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GLHelper assumes that the texture used in readback with it is always
GL_TEXTURE_2D which is not the case for resources creates for canvas2d
elements on mac using GL_TEXTURE_RECTANGLE_ARB.

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

Bug: 1020796
Change-Id: Ic5a4b8e0ba5a0d61a8af43251438f5323b911fd5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1902506
Commit-Queue: Khushal <khushalsagar@chromium.org>
Reviewed-by: Miguel Casas <mcasas@chromium.org>
Reviewed-by: Eric Karl <ericrk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713649}