Use StringPiece for the optional suffix in base::GetUniquePathNumber.

This removes the need to create/destroy a string when calling. This
change also adds a default value of an empty StringPiece to the suffix
so that callers who don't care about a suffix don't need to bother with
it. Finally, this change adds a unit test for the function, which it
apparently never had.

BUG=none
TBR=vasilii@chromium.org,vakh@chromium.org,eladalon@chromium.org,jamiewalch@chromium.org,carlosk@chromium.org

Change-Id: Iaf98d5237ffcc211bf9dfc2242742508f35bafac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1752302
Commit-Queue: Greg Thompson <grt@chromium.org>
Auto-Submit: Greg Thompson <grt@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#686754}

• base/files/file_util.cc[diff]
• base/files/file_util.h[diff]
• base/files/file_util_unittest.cc[diff]
• chrome/browser/media/webrtc/webrtc_event_log_manager_local.cc[diff]
• chrome/browser/safe_browsing/test_safe_browsing_database_helper.cc[diff]
• chrome/browser/web_applications/components/web_app_shortcut_win.cc[diff]
• components/offline_pages/core/model/offline_page_model_utils.cc[diff]
• remoting/host/file_transfer/local_file_operations.cc[diff]