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On Win/Linux, we will fallback to rendering with SwiftShader.
On other platforms, we will fallback to software rendering with no gpu process.

BUG=783069
TEST=the added tests
R=kbr@chromium.org

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: I9a60c71fba04ebdb46d92a5a0442a75f4f236e90
Reviewed-on: https://chromium-review.googlesource.com/764000Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Zhenyao Mo <zmo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#515780}