"platform.h" is too common a name and causes headers to be
included incorrectly. Disambiguate the header using a more
specific name.

Solves a problem that came up with the GLES 1 tests and the
standalone test harness.

Bug: angleproject:3162
Change-Id: I88229a2c9407e0db57f5beee44daa11a4075f700
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2229065
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>