cc: Glue LTHI and Scheduler changes for checker-imaging.

This change glues together the scheduling and tile management changes
for checker-imaging via ProxyImpl and SingleThreadProxy for enabling
checker-imaging. And adds integration LayerTreeTests, including a
scroll test to ensure that an impl-side pending tree does not affect
SyncedProperty state synchronization.

BUG=686267, 691041
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel

Review-Url: https://codereview.chromium.org/2717553005
Cr-Commit-Position: refs/heads/master@{#453508}

diff --git a/cc/BUILD.gn b/cc/BUILD.gn
index e0d8f256..a966184 100644
--- a/cc/BUILD.gn
+++ b/cc/BUILD.gn

@@ -853,6 +853,7 @@
     "trees/layer_tree_host_pixeltest_tiles.cc",
     "trees/layer_tree_host_unittest.cc",
     "trees/layer_tree_host_unittest_animation.cc",
+    "trees/layer_tree_host_unittest_checkerimaging.cc",
     "trees/layer_tree_host_unittest_context.cc",
     "trees/layer_tree_host_unittest_copyrequest.cc",
     "trees/layer_tree_host_unittest_damage.cc",