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This converts STGLO to be a pure virtual interface so we can mock it.
It will be used in a coming CL.

Bug: 660942
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: Ief880307a7ea47d9284fdd26f624de6bf39a6c86
Reviewed-on: https://chromium-review.googlesource.com/520662Reviewed-by: John Bauman <jbauman@chromium.org>
Reviewed-by: Frank Liberato <liberato@chromium.org>
Commit-Queue: Chris Watkins <watk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#476537}