Extend more runtime envs for generateMipmap srgb emulation

Add entries for windows, chromeos, and linux mesa in gpu_driver_bug_list.
Generate shaders according to different gl contexts.

BUG=634519

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/2548363002
Cr-Commit-Position: refs/heads/master@{#437398}
4 files changed