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

cc/trees/layer_tree_host_impl.cc

 // Copyright 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/trees/layer_tree_host_impl.h"
 
 #include <algorithm>
 #include <limits>
 
 #include "base/basictypes.h"
@@ skipping 2473 matching lines @@
       scroll_layer_id_when_mouse_over_scrollbar_ = 0;
     }
 
     return true;
   }
 
   return false;
 }
 
 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();
   pinch_gesture_end_should_clear_scrolling_layer_ = !CurrentlyScrollingLayer();
   if (active_tree_->OuterViewportScrollLayer()) {
     active_tree_->SetCurrentlyScrollingLayer(
         active_tree_->OuterViewportScrollLayer());
   } else {
     active_tree_->SetCurrentlyScrollingLayer(
         active_tree_->InnerViewportScrollLayer());
   }
   if (top_controls_manager_)
     top_controls_manager_->PinchBegin();
 }
 
 void LayerTreeHostImpl::PinchGestureUpdate(float magnify_delta,
                                            gfx::Point anchor) {
+  if (!InnerViewportScrollLayer())
+    return;
+
   TRACE_EVENT0("cc", "LayerTreeHostImpl::PinchGestureUpdate");
 
-  DCHECK(InnerViewportScrollLayer());
-
   // 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();
   gfx::PointF previous_scale_anchor =
       gfx::ScalePoint(anchor, 1.f / page_scale_delta);
   active_tree_->SetPageScaleDelta(page_scale_delta * magnify_delta);
   page_scale_delta = active_tree_->page_scale_delta();
   gfx::PointF new_scale_anchor =
       gfx::ScalePoint(anchor, 1.f / page_scale_delta);
   gfx::Vector2dF move = previous_scale_anchor - new_scale_anchor;
@@ skipping 19 matching lines @@
     InnerViewportScrollLayer()->ScrollBy(unused);
     InnerViewportScrollLayer()->ClampScrollToMaxScrollOffset();
   }
 
   client_->SetNeedsCommitOnImplThread();
   SetNeedsRedraw();
   client_->RenewTreePriority();
 }
 
 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;
     ClearCurrentlyScrollingLayer();
   }
   if (top_controls_manager_)
     top_controls_manager_->PinchEnd();
   client_->SetNeedsCommitOnImplThread();
 }
 
@@ skipping 434 matching lines @@
   swap_promise_monitor_.erase(monitor);
 }
 
 void LayerTreeHostImpl::NotifySwapPromiseMonitorsOfSetNeedsRedraw() {
   std::set<SwapPromiseMonitor*>::iterator it = swap_promise_monitor_.begin();
   for (; it != swap_promise_monitor_.end(); it++)
     (*it)->OnSetNeedsRedrawOnImpl();
 }
 
 }  // namespace cc