Add cc::LayerClient::OnLayerOpacityChanged.

This method is called when the opacity of a cc::Layer changes.

It is used to ensure that ui::LayerDelegate::OnLayerOpacityChanged()
is called even when the opacity of a layer changes without going
through ui::Layer::SetOpacityFromAnimation().

TBR=junov@chromium.org

Bug: 728208
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I0d7055591748c824687b93700aec9d38045b7c9e
Reviewed-on: https://chromium-review.googlesource.com/671116
Commit-Queue: Francois Doray <fdoray@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#502994}

• cc/BUILD.gn[diff]
• cc/layers/layer.cc[diff]
• cc/layers/layer.h[diff]
• cc/layers/layer_client.h[diff]
• cc/layers/layer_unittest.cc[diff]
• cc/test/mock_layer_client.cc[Added - diff]
• cc/test/mock_layer_client.h[Added - diff]
• cc/trees/layer_tree_host_unittest_animation.cc[diff]
• third_party/WebKit/Source/platform/graphics/GraphicsLayer.h[diff]
• ui/compositor/layer.cc[diff]
• ui/compositor/layer.h[diff]