Since libGLESv2 and libEGL will eventually be cross platform, it will be
useful to have platform defines and TLS functions that work everywhere.

BUG=angle:664

Change-Id: Ia357925a0992d82e8b446d88d32a1984d319e6e8
Reviewed-on: https://chromium-review.googlesource.com/202133Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>