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This change applies the existing gpu bug workaround,
init_gl_position_in_vertex_shader, to devices with PowerVR
GPUs. By initializing gl_postion to a known value, a vertex
shader can be created. This allows the following gl_tests
to pass on PowerVR devices:

GLOOBAttribTest.DrawUsingOOBMatrixAttrib

R=piman@chromium.org

TEST=Existing tests pass

Bug: 286468, 416670
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: Iecb91777d093cdd0eb8079dc56aca92f6e4ad93d
Reviewed-on: https://chromium-review.googlesource.com/956531Reviewed-by: Antoine Labour <piman@chromium.org>
Commit-Queue: Antoine Labour <piman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543279}