To support ANGLE for Android need a way to tell ANGLE
what library to call when using GL backend.

Bug: angleproject:2418
Test: test-opengl-gl2_copyTexImage
Change-Id: Ie00251b0672a036db4720e1e5b269f66a30a03da
Reviewed-on: https://chromium-review.googlesource.com/1066822
Commit-Queue: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Frank Henigman <fjhenigman@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>