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Fix two issues with GetSurfaceSize(). First, the Surface could have been
evicted so check if |current_surface| is null. Second, simplify logic by
getting the size_in_pixels() from Surface::surface_info_ since it will
always be the same for active/fallback frame.

Bug: none
Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel
Change-Id: If38f0673c878ebaadba8ee7038b4f677a522bdbe
Reviewed-on: https://chromium-review.googlesource.com/783253Reviewed-by: Yuri Wiitala <miu@chromium.org>
Commit-Queue: kylechar <kylechar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518452}