Re-land "Export angle::PlatformMethods." 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>