Replace GLES2Decoder callbacks by a GLES2DecoderClient interface

This reduces the need to test for callbacks (always set in production) and
significantly simplifies initialization code.

Bug: 723770
Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I8d702171a28748d5bd27a5cb228b1589c7575298
Reviewed-on: https://chromium-review.googlesource.com/524895
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Commit-Queue: Antoine Labour <piman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#477488}
33 files changed