Clear SVG property synchronization flag on attribute update

When an SVGAnimatedProperty has its associated attribute updated, we can
cancel any pending attribute synchronization.
This prevents overwriting the new value with the old in some cases.

Bug: 843901
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I3654ad616fd6e6ef82a1933ec034bc9cf0097d56
Reviewed-on: https://chromium-review.googlesource.com/1189842
Reviewed-by: Stephen Chenney <schenney@chromium.org>
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Cr-Commit-Position: refs/heads/master@{#586246}
3 files changed