[base] Prepare base/files for base::string16 switch

This change prepares base/files for the base::string16 switch to
std::u16string.

Bug: 911896
Change-Id: Ieb039f1394419291b9c710023be9e8190d3f2c54
Reviewed-on: https://chromium-review.googlesource.com/c/1437281
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#626984}

• base/files/file_enumerator_win.cc[diff]
• base/files/file_path.cc[diff]
• base/files/file_path_unittest.cc[diff]
• base/files/file_path_watcher_win.cc[diff]
• base/files/file_unittest.cc[diff]
• base/files/file_util.cc[diff]
• base/files/file_util_unittest.cc[diff]
• base/files/file_util_win.cc[diff]
• base/files/file_win.cc[diff]
• base/strings/string16.cc[diff]
• base/strings/string_piece.cc[diff]
• base/strings/string_piece.h[diff]