GitLab

Since GPU raster uses the GpuImageDecodeCache for image uploads,
ensuring that the image is tracked as discardable avoids at-raster
caching for them which has more ref/unref overhead.

R=ericrk@chromium.org

Bug: 894200
Change-Id: I9d40ab5e0d7c2c2e4a12d7ab8be318cfcba2f590
Reviewed-on: https://chromium-review.googlesource.com/c/1292813Reviewed-by: Eric Karl <ericrk@chromium.org>
Commit-Queue: Khushal <khushalsagar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601717}