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This is more consistent overall, GetGLImplementation() doesn't tell exactly
whether we're using the core profile, or which version of ES etc.
It also helps testing & fuzzing, because mocks/stubs may implement ES2 but don't
report kGLImplementationEGLGLES2, causing inconsistencies.

BUG=None
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/2170293002
Cr-Commit-Position: refs/heads/master@{#407276}