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LayerImpl has IsActive function to check if layer is on active tree.
Use that function instead of the PictureLayerImpl's GetTree function.

R=danakj

Bug: 822873
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.android:android_optional_gpu_tests_rel
Change-Id: I169e9ccbca359cc56cb48a7b3587d8bd2f5b753c
Reviewed-on: https://chromium-review.googlesource.com/967268
Commit-Queue: weiliangc <weiliangc@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543951}