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This CL fixes a bug where LayerTreeHostImpl::OnPurgeMemory() causes
TextureLayerImpl to register duplicate SharedBitmapIds. OnPurgeMemory()
ends up calling TextureLayerImpl::ReleaseResources(), which assumed that
the LayerTreeFrameSink was lost if it was called. SharedBitmapIds should
only be reregistered if the LayerTreeFrameSink is actually lost.

Add new OnPurgeMemory() function on LayerImpl. This will by default keep
the old behaviour by calling ReleaseResources() but TextureLayerImpl can
override it to not do anything.

Bug: 862584
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I32cd4e45d629b2f8410adff80f712f1b517a1407
Reviewed-on: https://chromium-review.googlesource.com/1169430Reviewed-by: weiliangc <weiliangc@chromium.org>
Commit-Queue: kylechar <kylechar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582045}