Reviewers: bokan, aelias
CL:
https://codereview.chromium.org/2842553003/Description:
Call SAC::DidScrollUpdate only for compositor-triggered scrolls.
For everything else we should rely on SAC::DidRequestShowFromMainThread.
Additionally, ensure that LayerTreeImpl::ShowScrollbars is called in synchronous
compositing mode (which commits directly to the active tree).
BUG=606395
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel
Affected files (+65, -114 lines):
M cc/layers/scrollbar_layer_unittest.cc
M cc/trees/layer_tree_host_impl.h
M cc/trees/layer_tree_host_impl.cc
M cc/trees/layer_tree_host_impl_unittest.cc
M cc/trees/layer_tree_impl.h
M cc/trees/layer_tree_impl.cc
M third_party/WebKit/LayoutTests/paint/invalidation/border-radius-repaint-2.html
M third_party/WebKit/LayoutTests/paint/invalidation/border-radius-repaint-2-expected.png
M third_party/WebKit/LayoutTests/paint/invalidation/window-resize-no-layout-change1-expected.html
M third_party/WebKit/LayoutTests/paint/invalidation/window-resize-no-layout-change2-expected.html