Added early out for pinch to prevent crash when no inner scroll layer.

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 ec89ddcfd734a055a9cdc9fa4dd9bc60bc870f99..d6e35a2dda279d5b00f1eca514ad7a40799bf4bd 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -2491,6 +2491,9 @@ bool LayerTreeHostImpl::HandleMouseOverScrollbar(LayerImpl* layer_impl,
 }
 
 void LayerTreeHostImpl::PinchGestureBegin() {
+  if (!InnerViewportScrollLayer())

[wjmaclean, 2014/02/18 21:16:59]

Do we really need early outs in PinchGestureBegin()/PinchGestureEnd()? Having
three checks seems excessive, since the original code only checked in the
PinchGestureUpdate() function, and that's the only place (I think) a problem can
actually occur.

[bokan, 2014/02/18 22:12:11]

I think it's safer/cleaner not to have any side effects if we're not actually
performing the pinch - but I've tried it without and it doesn't crash (at least
in the limited testing I've done) so I've removed them on Begin/End. 

+    return;
+
   pinch_gesture_active_ = true;
   previous_pinch_anchor_ = gfx::Point();
   client_->RenewTreePriority();
@@ -2508,9 +2511,10 @@ void LayerTreeHostImpl::PinchGestureBegin() {
 
 void LayerTreeHostImpl::PinchGestureUpdate(float magnify_delta,
                                            gfx::Point anchor) {
-  TRACE_EVENT0("cc", "LayerTreeHostImpl::PinchGestureUpdate");
+  if (!InnerViewportScrollLayer())
+    return;
 
-  DCHECK(InnerViewportScrollLayer());
+  TRACE_EVENT0("cc", "LayerTreeHostImpl::PinchGestureUpdate");
 
   // Keep the center-of-pinch anchor specified by (x, y) in a stable
   // position over the course of the magnify.
@@ -2551,6 +2555,9 @@ void LayerTreeHostImpl::PinchGestureUpdate(float magnify_delta,
 }
 
 void LayerTreeHostImpl::PinchGestureEnd() {
+  if (!InnerViewportScrollLayer())
+    return;
+
   pinch_gesture_active_ = false;
   if (pinch_gesture_end_should_clear_scrolling_layer_) {
     pinch_gesture_end_should_clear_scrolling_layer_ = false;