cc: Fix for synced property main thread updates with MFBA.

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 7d1e563343ab8553c26930ae690a8b18de929787..26b00d49b55efaf18a55a6c12cd208505d040b6c 100644
--- a/cc/base/synced_property.h
+++ b/cc/base/synced_property.h
@@ -57,18 +57,20 @@ class SyncedProperty : public base::RefCounted<SyncedProperty<T>> {
   // Returns the latest active tree delta and also makes a note that this value
   // was sent to the main thread.
   typename T::ValueType PullDeltaForMainThread() {
-    sent_delta_ = active_delta_;
-    return active_delta_.get();
+    sent_delta_ = PendingDelta();
+    return sent_delta_.get();
   }
 
   // Push the latest value from the main thread onto pending tree-associated
   // state.  Returns true if this had any effect.
   bool PushFromMainThread(typename T::ValueType main_thread_value) {
+    pending_delta_ = sent_delta_;
+    sent_delta_ = T::Identity();
+
     if (pending_base_.get() == main_thread_value)
       return false;
 
     pending_base_ = T(main_thread_value);
-
     return true;
   }
 
@@ -78,12 +80,12 @@ class SyncedProperty : public base::RefCounted<SyncedProperty<T>> {
   // only the difference since the last send).
   bool PushPendingToActive() {
     if (active_base_.get() == pending_base_.get() &&
-        sent_delta_.get() == T::Identity().get())
+        pending_delta_.get() == T::Identity().get())
       return false;
 
     active_base_ = pending_base_;
     active_delta_ = PendingDelta();
-    sent_delta_ = T::Identity();
+    pending_delta_ = T::Identity();
     clobber_active_value_ = false;
 
     return true;
@@ -94,7 +96,7 @@ class SyncedProperty : public base::RefCounted<SyncedProperty<T>> {
   // active value, and the sent_delta is subtracted from the delta.
   void AbortCommit() {
     active_base_ = active_base_.Combine(sent_delta_);
-    active_delta_ = PendingDelta();
+    active_delta_ = active_delta_.InverseCombine(sent_delta_);
     sent_delta_ = T::Identity();
   }
 
@@ -109,7 +111,7 @@ class SyncedProperty : public base::RefCounted<SyncedProperty<T>> {
   T PendingDelta() const {

[aelias_OOO_until_Jul13, 2016/06/07 23:52:26]

It's really confusing that this method doesn't return the value of
pending_delta_ -- it needs a different name.  I see this is now the return value
of PullDeltaForMainThread(), so how about calling this DeltaForMainThread()?

     if (clobber_active_value_)
       return T::Identity();
-    return active_delta_.InverseCombine(sent_delta_);
+    return active_delta_.InverseCombine(pending_delta_);
   }
 
   void set_clobber_active_value() { clobber_active_value_ = true; }
@@ -118,9 +120,11 @@ class SyncedProperty : public base::RefCounted<SyncedProperty<T>> {
  private:
   // Value last committed to the pending tree.
   T pending_base_;
+  // The delta that the main thread saw for the pending tree.
+  T pending_delta_;
   // Value last committed to the active tree (on the last activation).
   T active_base_;
-  // The difference between the active_base_ and the user-perceived value.
+  // The difference between |active_base_| and the user-perceived value.
   T active_delta_;
   // The value sent to the main thread (on the last BeginFrame); this is always
   // identity outside of the BeginFrame-to-activation interval.