cc: Move pre-decodes to background worker.

cc/trees/layer_tree_host_unittest_checkerimaging.cc

Index: cc/trees/layer_tree_host_unittest_checkerimaging.cc
diff --git a/cc/trees/layer_tree_host_unittest_checkerimaging.cc b/cc/trees/layer_tree_host_unittest_checkerimaging.cc
index 692bb747cebc75956dfbdbc1f9ade15cbab2bc44..7dd7bdcd2b213a6ac939e1ec20e04da2ba4c6066 100644
--- a/cc/trees/layer_tree_host_unittest_checkerimaging.cc
+++ b/cc/trees/layer_tree_host_unittest_checkerimaging.cc
@@ -81,18 +81,30 @@ class LayerTreeHostCheckerImagingTestMergeWithMainFrame
     }
   }
 
+  void NotifyAllTileTasksCompleted(LayerTreeHostImpl* host_impl) override {
+    // Wait until all tile tasks have finished before flushing the decode tasks.
+    // The CheckerImageTracker does not queue decode work until higher priority
+    // raster work has finished.
+
+    // Run all tasks on the image worker to ensure that the decode completion
+    // triggers an impl-side invalidation request.
+    FlushImageDecodeTasks();
+
+    // Request a commit to ensure that this invalidation is merged with the
+    // incoming frame.
+    host_impl->GetTaskRunner()->PostTask(
+        FROM_HERE, base::Bind(&LayerTreeHostImpl::SetNeedsCommit,
+                              base::Unretained(host_impl)));
+  }
+
   void CommitCompleteOnThread(LayerTreeHostImpl* host_impl) override {
     switch (++num_of_commits_) {
-      case 1: {
-        // The first commit has happened. Run all tasks on the image worker to
-        // ensure that the decode completion triggers an impl-side invalidation
-        // request.
-        FlushImageDecodeTasks();
-
-        // Block notifying the scheduler of this request until we get a commit.
+      case 1:
+        // Block notifying the scheduler of this request until we've had a
+        // chance to make sure that the decode work was scheduled, flushed and
+        // the commit requested after it is received.
         host_impl->BlockImplSideInvalidationRequestsForTesting(true);
-        host_impl->SetNeedsCommit();
-      } break;
+        break;
       case 2: {
         // Ensure that the expected tiles are invalidated on the sync tree.
         PictureLayerImpl* sync_layer_impl = static_cast<PictureLayerImpl*>(
@@ -102,7 +114,9 @@ class LayerTreeHostCheckerImagingTestMergeWithMainFrame
                 ->FindTilingWithResolution(TileResolution::HIGH_RESOLUTION);
 
         for (int i = 0; i < 4; i++) {
+          SCOPED_TRACE(i);
           for (int j = 0; j < 2; j++) {
+            SCOPED_TRACE(j);
             Tile* tile =
                 sync_tiling->TileAt(i, j) ? sync_tiling->TileAt(i, j) : nullptr;