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This moves the |base_value_updated_| flag from the "primitive type"
version of SVGAnimatedProperty to SVGAnimatedPropertyBase, and sets it
in BaseValueChanged. The flag is cleared in SynchronizeAttribute.
This allows some code to be simplified. It also avoids synchronizations
triggered by just having created the tear-off object like in the bug.

Bug: 873470
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I59f30dd69286dc0799a583eb6a52c41022b0af91
Reviewed-on: https://chromium-review.googlesource.com/1188305
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585852}