Update from chromium https://crrev.com/302282

cc/trees/layer_tree_host.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.h"
 
 #include <algorithm>
 #include <stack>
 #include <string>
 
@@ skipping 1040 matching lines @@
           it->IsSuitableForGpuRasterization();
     }
 
     occlusion_tracker.LeaveLayer(it);
   }
 
   in_paint_layer_contents_ = false;
 }
 
 void LayerTreeHost::ApplyScrollAndScale(ScrollAndScaleSet* info) {
-  if (!root_layer_.get())
-    return;
-
   ScopedPtrVector<SwapPromise>::iterator it = info->swap_promises.begin();
   for (; it != info->swap_promises.end(); ++it) {
     scoped_ptr<SwapPromise> swap_promise(info->swap_promises.take(it));
     TRACE_EVENT_FLOW_STEP0("input",
                            "LatencyInfo.Flow",
                            TRACE_ID_DONT_MANGLE(swap_promise->TraceId()),
                            "Main thread scroll update");
     QueueSwapPromise(swap_promise.Pass());
   }
 
   gfx::Vector2d inner_viewport_scroll_delta;
   gfx::Vector2d outer_viewport_scroll_delta;
 
-  for (size_t i = 0; i < info->scrolls.size(); ++i) {
-    Layer* layer = LayerTreeHostCommon::FindLayerInSubtree(
-        root_layer_.get(), info->scrolls[i].layer_id);
-    if (!layer)
-      continue;
-    if (layer == outer_viewport_scroll_layer_.get()) {
-      outer_viewport_scroll_delta += info->scrolls[i].scroll_delta;
-    } else if (layer == inner_viewport_scroll_layer_.get()) {
-      inner_viewport_scroll_delta += info->scrolls[i].scroll_delta;
-    } else {
-      layer->SetScrollOffsetFromImplSide(
-          gfx::ScrollOffsetWithDelta(layer->scroll_offset(),
-                                     info->scrolls[i].scroll_delta));
+  if (root_layer_.get()) {
+    for (size_t i = 0; i < info->scrolls.size(); ++i) {
+      Layer* layer = LayerTreeHostCommon::FindLayerInSubtree(
+          root_layer_.get(), info->scrolls[i].layer_id);
+      if (!layer)
+        continue;
+      if (layer == outer_viewport_scroll_layer_.get()) {
+        outer_viewport_scroll_delta += info->scrolls[i].scroll_delta;
+      } else if (layer == inner_viewport_scroll_layer_.get()) {
+        inner_viewport_scroll_delta += info->scrolls[i].scroll_delta;
+      } else {
+        layer->SetScrollOffsetFromImplSide(
+            gfx::ScrollOffsetWithDelta(layer->scroll_offset(),
+                                       info->scrolls[i].scroll_delta));
+      }
     }
   }
 
   if (!inner_viewport_scroll_delta.IsZero() ||
       !outer_viewport_scroll_delta.IsZero() ||
       info->page_scale_delta != 1.f ||
       info->top_controls_delta) {
-    // SetScrollOffsetFromImplSide above could have destroyed the tree,
-    // so re-get this layer before doing anything to it.
-
-    DCHECK(inner_viewport_scroll_layer_.get());  // We should always have this.
-
     // Preemptively apply the scroll offset and scale delta here before sending
     // it to the client.  If the client comes back and sets it to the same
     // value, then the layer can early out without needing a full commit.
-    inner_viewport_scroll_layer_->SetScrollOffsetFromImplSide(
-        gfx::ScrollOffsetWithDelta(
-            inner_viewport_scroll_layer_->scroll_offset(),
-            inner_viewport_scroll_delta));
+    if (inner_viewport_scroll_layer_.get()) {
+      inner_viewport_scroll_layer_->SetScrollOffsetFromImplSide(
+          gfx::ScrollOffsetWithDelta(
+              inner_viewport_scroll_layer_->scroll_offset(),
+              inner_viewport_scroll_delta));
+    }
+
     if (outer_viewport_scroll_layer_.get()) {
       outer_viewport_scroll_layer_->SetScrollOffsetFromImplSide(
           gfx::ScrollOffsetWithDelta(
               outer_viewport_scroll_layer_->scroll_offset(),
               outer_viewport_scroll_delta));
     }
 
     ApplyPageScaleDeltaFromImplSide(info->page_scale_delta);
-    if (!outer_viewport_scroll_layer_.get()) {
+    if (!settings_.use_pinch_virtual_viewport) {
       client_->ApplyViewportDeltas(
           inner_viewport_scroll_delta + outer_viewport_scroll_delta,
           info->page_scale_delta,
           info->top_controls_delta);
     } else {
       client_->ApplyViewportDeltas(
           inner_viewport_scroll_delta,
           outer_viewport_scroll_delta,
           info->page_scale_delta,
           info->top_controls_delta);
@@ skipping 205 matching lines @@
   swap_promise_list_.push_back(swap_promise.Pass());
 }
 
 void LayerTreeHost::BreakSwapPromises(SwapPromise::DidNotSwapReason reason) {
   for (size_t i = 0; i < swap_promise_list_.size(); i++)
     swap_promise_list_[i]->DidNotSwap(reason);
   swap_promise_list_.clear();
 }
 
 }  // namespace cc