Make LayerScrollOffsetDelegate updates consistent.

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index e158759cc21ef7ac575b6bec2ffeb86aa9490a65..d00faf0e8240e367efee64f1476c034720d79d1e 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -2685,6 +2685,11 @@ void LayerTreeHostImpl::PinchGestureUpdate(float magnify_delta,
 
   TRACE_EVENT0("cc", "LayerTreeHostImpl::PinchGestureUpdate");
 
+  // For a moment the scroll offset ends up being outside of the max range. This
+  // confuses the delegate so we switch it off till after we're done processing
+  // the pinch update.
+  active_tree_->SetRootLayerScrollOffsetDelegate(NULL);

[aelias_OOO_until_Jul13, 2014/05/09 01:00:01]

Is there any way to avoid this workaround by changing the order that things
happen in the scrolling guts?  I don't understand quite what the problem is from
code inspection, I guess either a clamp or a max scroll offset update happens
too late?

[mkosiba (inactive), 2014/05/09 17:05:33]

AFAICT the problem is caused by 2 things:
a) the LayerImpl::MaxScrollOffset seems to now depend on pagescale,
b) updating the scroll offset on one of the viewport causes an update on the
delegate.

I think the above makes it quite hard to change the scroll offset in a way that
doesn't result in the delegate seeing an intermediate value (even though it
might be possible to change this in a way that makes the delegate not see a
value that's outside of the max range).

+
   // Keep the center-of-pinch anchor specified by (x, y) in a stable
   // position over the course of the magnify.
   float page_scale_delta = active_tree_->page_scale_delta();
@@ -2718,6 +2723,9 @@ void LayerTreeHostImpl::PinchGestureUpdate(float magnify_delta,
     InnerViewportScrollLayer()->ClampScrollToMaxScrollOffset();
   }
 
+  active_tree_->SetRootLayerScrollOffsetDelegate(
+      root_layer_scroll_offset_delegate_);
+
   client_->SetNeedsCommitOnImplThread();
   SetNeedsRedraw();
   client_->RenewTreePriority();