Implement scroll handler latency tracking

cc/trees/layer_tree_host_unittest.cc

Index: cc/trees/layer_tree_host_unittest.cc
diff --git a/cc/trees/layer_tree_host_unittest.cc b/cc/trees/layer_tree_host_unittest.cc
index b194491d7606763193f75e034c965c5b022ebeb7..941ccea207f9f3ba5fa248ad94e80eb36399bdfc 100644
--- a/cc/trees/layer_tree_host_unittest.cc
+++ b/cc/trees/layer_tree_host_unittest.cc
@@ -4517,8 +4517,7 @@ class SimpleSwapPromiseMonitor : public SwapPromiseMonitor {
                            int* set_needs_commit_count,
                            int* set_needs_redraw_count)
       : SwapPromiseMonitor(layer_tree_host, layer_tree_host_impl),
-        set_needs_commit_count_(set_needs_commit_count),
-        set_needs_redraw_count_(set_needs_redraw_count) {}
+        set_needs_commit_count_(set_needs_commit_count) {}
 
   virtual ~SimpleSwapPromiseMonitor() {}
 
@@ -4527,12 +4526,15 @@ class SimpleSwapPromiseMonitor : public SwapPromiseMonitor {
   }
 
   virtual void OnSetNeedsRedrawOnImpl() OVERRIDE {
-    (*set_needs_redraw_count_)++;
+    ADD_FAILURE() << "Should not get called on main thread.";
+  }
+
+  virtual void OnForwardScrollUpdateToMainThreadOnImpl() OVERRIDE {
+    ADD_FAILURE() << "Should not get called on main thread.";
   }
 
  private:
   int* set_needs_commit_count_;
-  int* set_needs_redraw_count_;
 };
 
 class LayerTreeHostTestSimpleSwapPromiseMonitor : public LayerTreeHostTest {