Removing one usage of FormatString() and its static buffer.
And preparation for Platform logging.

BUG=angleproject:1660

Change-Id: I58192988ad16196706fe48d0c0ab0fd1a10c0210
Reviewed-on: https://chromium-review.googlesource.com/424173Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Yuly Novikov <ynovikov@chromium.org>