set_sources_assignment_filter() is considered as a mis-feature
of gn (as it makes it difficult to reason about the BUILD.gn
files individually).

Change libGLESv2.gni in angle to check the platform before
adding plaform-specific source files to "sources" variable.
This will allow removing the global sources assignment filter
from Chromium codebase.

Bug: chromium:1018739
Change-Id: I5db637a583d9cf547f6f175baa445d47751e6a3c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2428763Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>