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

Fix incorrect enabling of ERR() calls in UNIMPLEMENTED() and
UNREACHABLE(), resulting in increased code size and
<iostream> adding 5 static initializers to chrome because of
cerr referenced in statically linked translator.

BUG=angleproject:1660

Change-Id: I7caa18036118d532e0544f75278602559172ae04
Reviewed-on: https://chromium-review.googlesource.com/431457Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>