ImportantFileWriter: Fix races part 1.

This CL renames CreateAndOpenTemporaryFile{,InDir} to
CreateAndOpenTemporaryStream{,InDir} and changes the return type to a
ScopedFILE. As a result, the name "CreateAndOpenTemporaryFile" is now
available for a future function that returns a base::File.

BUG=1075917
TBR=jam@chromium.org

Change-Id: Ib79613ed63e552df5c13d2e4e7078e0c16b42b5d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2166175
Commit-Queue: Greg Thompson <grt@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#764175}

diff --git a/base/files/file_util.cc b/base/files/file_util.cc
index 5ecb877d..430321b 100644
--- a/base/files/file_util.cc
+++ b/base/files/file_util.cc

@@ -202,12 +202,12 @@
   return false;
 }
 
-FILE* CreateAndOpenTemporaryFile(FilePath* path) {
+ScopedFILE CreateAndOpenTemporaryStream(FilePath* path) {
   FilePath directory;
   if (!GetTempDir(&directory))
     return nullptr;
 
-  return CreateAndOpenTemporaryFileInDir(directory, path);
+  return CreateAndOpenTemporaryStreamInDir(directory, path);
 }
 
 bool CreateDirectory(const FilePath& full_path) {