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}

diff --git a/cc/BUILD.gn b/cc/BUILD.gn
index 8555592..b1414ef 100644
--- a/cc/BUILD.gn
+++ b/cc/BUILD.gn

@@ -458,6 +458,8 @@
     "test/layer_tree_pixel_test.h",
     "test/layer_tree_test.cc",
     "test/layer_tree_test.h",
+    "test/mock_layer_client.cc",
+    "test/mock_layer_client.h",
     "test/mock_occlusion_tracker.h",
     "test/ordered_texture_map.cc",
     "test/ordered_texture_map.h",