[Windows] Do not include string terminator in GetWindowText

WindowEnumerator::GetWindowText was including the trailing string
terminator in the body of the returned string. This CL fixes that as
well as makes several other changes:

- WindowEnumeratorTest.EnumerateTopLevelWindows had the same bug in its
  independent implementation of GetWindowText. It is also fixed.
- WindowEnumerator has been whittled down to a single function, so
  there's no need for a class anymore. It's now simply
  base::win::EnumerateChildWindows, which is a little easier to use.
- base/win/window_enumerator.h no longer includes windows.h.
- GetWindowText has been renamed to GetWindowTextString so that it
  doesn't accidentally get renamed to GetWindowTextW by windows.h.
- GetWindowTextString also handles the case where the actual text is
  fewer characters than reported by GetWindowTextLength. MSDN explains a
  few cases where this could happen.

Bug: 1482637
Change-Id: I8cb9b5706fb43bdb10e79afc9c832d9a4e5ee6af
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4868390
Auto-Submit: Greg Thompson <grt@chromium.org>
Reviewed-by: S Ganesh <ganesh@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1197669}

diff --git a/base/win/window_enumerator.h b/base/win/window_enumerator.h
index ee68ce40..663df52 100644
--- a/base/win/window_enumerator.h
+++ b/base/win/window_enumerator.h

@@ -5,54 +5,34 @@
 #ifndef BASE_WIN_WINDOW_ENUMERATOR_H_
 #define BASE_WIN_WINDOW_ENUMERATOR_H_
 
-#include <windows.h>
-
 #include <string>
 
 #include "base/base_export.h"
 #include "base/functional/callback.h"
+#include "base/win/windows_types.h"
 
 namespace base::win {
 
-// Enumerates immediate child windows of `parent`, and calls `filter.Run` for
-// each window:
-// * If `filter.Run` returns `false`, continues enumerating.
-// * If `filter.Run` returns `true`, stops enumerating.
-class BASE_EXPORT WindowEnumerator {
- public:
-  WindowEnumerator(HWND parent,
-                   base::RepeatingCallback<bool(HWND hwnd)> filter);
-  WindowEnumerator(const WindowEnumerator&) = delete;
-  WindowEnumerator& operator=(const WindowEnumerator&) = delete;
-  ~WindowEnumerator();
+// Enumerates immediate child windows of `parent`, running `filter` for each
+// window until `filter` returns true.
+using WindowEnumeratorCallback = base::RepeatingCallback<bool(HWND hwnd)>;
+BASE_EXPORT void EnumerateChildWindows(HWND parent,
+                                       WindowEnumeratorCallback filter);
 
-  void Run() const;
+// Returns true if `hwnd` is an always-on-top window.
+BASE_EXPORT bool IsTopmostWindow(HWND hwnd);
 
-  // Returns true if `hwnd` is an always-on-top window.
-  static bool IsTopmostWindow(HWND hwnd);
+// Returns true if `hwnd` is a system dialog.
+BASE_EXPORT bool IsSystemDialog(HWND hwnd);
 
-  // Returns true if `hwnd` is a system dialog.
-  static bool IsSystemDialog(HWND hwnd);
+// Returns true if `hwnd` is a window owned by the Windows shell.
+BASE_EXPORT bool IsShellWindow(HWND hwnd);
 
-  // Returns true if `hwnd` is a window owned by the Windows shell.
-  static bool IsShellWindow(HWND hwnd);
+// Returns the class name of `hwnd` or an empty string on error.
+BASE_EXPORT std::wstring GetWindowClass(HWND hwnd);
 
-  // Returns the class name of `hwnd` or an empty string on error.
-  static std::wstring GetWindowClass(HWND hwnd);
-
-  // Returns the window text for `hwnd`, or an empty string on error.
-  static std::wstring GetWindowText(HWND hwnd);
-
- private:
-  // Main processing function run for each window.
-  bool OnWindow(HWND hwnd) const;
-
-  // An EnumWindowsProc invoked by EnumWindows once for each window.
-  static BOOL CALLBACK OnWindowProc(HWND hwnd, LPARAM lparam);
-
-  const HWND parent_;
-  base::RepeatingCallback<bool(HWND hwnd)> filter_;
-};
+// Returns the window text for `hwnd`, or an empty string on error.
+BASE_EXPORT std::wstring GetWindowTextString(HWND hwnd);
 
 }  // namespace base::win