cc/blimp: Add synchronization for scroll/scale state.

cc/base/synced_property.h

Index: cc/base/synced_property.h
diff --git a/cc/base/synced_property.h b/cc/base/synced_property.h
index dfb33aa5ab2dfb859adeeccd652f983d693f88e2..b5354d84219a5e630eb4aae4a587a0ade19b5be3 100644
--- a/cc/base/synced_property.h
+++ b/cc/base/synced_property.h
@@ -50,6 +50,17 @@ class SyncedProperty : public base::RefCounted<SyncedProperty<T>> {
     return true;
   }
 
+  // Adds the delta that has been reported to the main thread, and was expected
+  // to be reflected/resolved by the main thread in the commit, but could not
+  // be applied because it has not yet been resolved by the LayerTreeHostClient.
+  // The delta is added to the |pending_base_| so while the current value on
+  // the associated pending/active tree state simulates the result of the delta
+  // having not been applied to the main thread, but it is excluded in the
+  // subsequent deltas reported to the main thread.
+  void AddUnappliedDeltaToPendingBase(typename T::ValueType unapplied_delta) {
+    pending_base_ = pending_base_.Combine(T(unapplied_delta));
+  }
+
   // Returns the difference between the last value that was committed and
   // activated from the main thread, and the current total value.
   typename T::ValueType Delta() const { return active_delta_.get(); }