Centralize definition of EGL_PLATFORM_ANGLE_PLATFORM_METHODS_ANGLEX
in Platform.h.

Bug: angleproject:2528
Change-Id: Id2b022c4ea08d2e98cbd6c3b248b58d096c20dfb
Reviewed-on: https://chromium-review.googlesource.com/1144265Reviewed-by: Tobin Ehlis <tobine@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>