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This cl enables experimental support for SwiftShader on MacOS. It is
still disabled by default since use_egl_on_mac is still false in
ui/gl/features.gni.

Bug: 726075
Cq-Include-Trybots: luci.chromium.try:linux_optional_gpu_tests_rel;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: I01093a2203a94d05e0588ff1deeb8016848d82b9
Reviewed-on: https://chromium-review.googlesource.com/955949
Commit-Queue: Alexis Hétu <sugoi@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544185}