Prep work for adding command buffer support for EXT_sRGB_write_control

This re-lands part of the work from crrev.com/435631, namely.

Move the logic for updating framebuffer SRGB in the decoder to
a separate function

TBR=zmo

BUG=655247
CQ_INCLUDE_TRYBOTS=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

Review-Url: https://codereview.chromium.org/2542343003
Cr-Commit-Position: refs/heads/master@{#435852}
4 files changed