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This CL moves the memory dump internals into SharedImageBacking, which
will allow for new backing types to implement their own dump logic.

Bug: 891059
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: I4f239ab76e75d3b51fe3a120e7c34b20376f559c
Reviewed-on: https://chromium-review.googlesource.com/c/1279167
Commit-Queue: Eric Karl <ericrk@chromium.org>
Reviewed-by: Jonathan Backer <backer@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600096}