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Calling Layer::set_property_tree_sequence_number() does not set the
flag that a commit is needed, which means it can quickly get out of
date for layers that are not updated for other reasons when the Layer
is (de)serialized.

This CL makes the LayerTreeHost forcefully set the sequence number
to the same value as the current PropertyTrees.sequence_number, since
all valid in the hierarchy is valid at that point.

BUG=577352, 538710
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1584653004

Cr-Commit-Position: refs/heads/master@{#371654}