This should fix undefined function call sanitizer warnings.

Re-land fixes the build and uses proper export annotation. Also fixes
the missing source includes in BUILD.gn for libANGLE so future
changes to Platform.h (and other includes) will get picked up by the
dependency analyzer step.

BUG=chromium:778918

Change-Id: I92cc8a43bbbdc70de50a67f73b223d8f903cd6cd
Reviewed-on: https://chromium-review.googlesource.com/767111
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>