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Use a common GrContext across all RasterDecoders. This ensures that we
get a unified GrProgramCache for compiled shaders.

R=backer@chromium.org, ericrk@chromium.org

Bug: 854416
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: I653486d2a20f4bb0112c6cfe66f3922c222ad338
Reviewed-on: https://chromium-review.googlesource.com/1114287
Commit-Queue: Khushal <khushalsagar@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Reviewed-by: Eric Karl <ericrk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572377}