Re-land: cc: Add raster finished signals to RasterWorkerPool.

cc/trees/layer_tree_host_unittest_scroll.cc

Index: cc/trees/layer_tree_host_unittest_scroll.cc
diff --git a/cc/trees/layer_tree_host_unittest_scroll.cc b/cc/trees/layer_tree_host_unittest_scroll.cc
index 8582d8f55a42a4bab09fee2324ec8da816b696ff..7e4dc366debc6fc4680d5d0fb8bf27670fceff97 100644
--- a/cc/trees/layer_tree_host_unittest_scroll.cc
+++ b/cc/trees/layer_tree_host_unittest_scroll.cc
@@ -571,6 +571,11 @@ class ImplSidePaintingScrollTestSimple : public ImplSidePaintingScrollTest {
     return can_activate_;
   }
 
+  virtual bool CanActivatePendingTreeIfNeeded(LayerTreeHostImpl* impl)
+      OVERRIDE {
+    return can_activate_;
+  }
+
   virtual void CommitCompleteOnThread(LayerTreeHostImpl* impl) OVERRIDE {
     // We force a second draw here of the first commit before activating
     // the second commit.