Two simple fixes to make libANGLE compile for Fuchsia:
1. Added stubs typedefs for platform-specific primitives in
   EGL/eglplatform.h for Fuchsia.
2. src/common/platform.h updated to define ANGLE_PLATFORM_POSIX on
   Fuchsia.

Change-Id: I5f053de46632ac3f12bb93e57eeb94c3a34c867c
Reviewed-on: https://chromium-review.googlesource.com/563758Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>