[base] Use std::wstring for native strings on Win

This change modifies the native strings used in base::FilePath,
base::CommandLine and base::Environment to be std::wstring on Windows.
Furthermore it updates callers within //base to be compatible with the
upcoming switch of base::string16 from std::wstring to std::u16string.

Bug: 911896
Change-Id: I3843e87c15ad0faeaa0e5e0ca3059c39e8108dd5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1817603
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Reviewed-by: Robert Liao <robliao@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#721421}

• base/base_paths_win.cc[diff]
• base/command_line.cc[diff]
• base/debug/activity_tracker.cc[diff]
• base/debug/stack_trace_win.cc[diff]
• base/file_version_info_win.cc[diff]
• base/file_version_info_win_unittest.cc[diff]
• base/files/os_validation_win_unittest.cc[diff]
• base/i18n/file_util_icu.cc[diff]
• base/i18n/file_util_icu_unittest.cc[diff]
• base/i18n/icu_util.cc[diff]
• base/i18n/rtl.cc[diff]
• base/i18n/rtl_unittest.cc[diff]
• base/i18n/string_compare.cc[diff]
• base/i18n/string_compare.h[diff]
• base/logging.cc[diff]
• base/logging.h[diff]
• base/native_library_win.cc[diff]
• base/process/process_iterator.h[diff]
• base/process/process_iterator_win.cc[diff]
• base/sampling_heap_profiler/module_cache_win.cc[diff]
• base/system/sys_info_win.cc[diff]
• base/test/launcher/test_launcher.cc[diff]
• base/test/test_file_util_win.cc[diff]
• base/test/test_shortcut_win.cc[diff]
• base/win/event_trace_consumer_unittest.cc[diff]
• base/win/event_trace_controller_unittest.cc[diff]
• base/win/scoped_process_information_unittest.cc[diff]
• base/win/shortcut.cc[diff]
• base/win/startup_information_unittest.cc[diff]
• base/win/win_util_unittest.cc[diff]