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

cc/trees/layer_tree_host.cc

Index: cc/trees/layer_tree_host.cc
diff --git a/cc/trees/layer_tree_host.cc b/cc/trees/layer_tree_host.cc
index b2c07da12191112f979efef8c435d386c240aae6..3898156776037a9420ffc8a9909eda0a49f3bcc9 100644
--- a/cc/trees/layer_tree_host.cc
+++ b/cc/trees/layer_tree_host.cc
@@ -1058,9 +1058,6 @@ void LayerTreeHost::PaintLayerContents(
 }
 
 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));
@@ -1074,19 +1071,21 @@ void LayerTreeHost::ApplyScrollAndScale(ScrollAndScaleSet* info) {
   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));
+      }
     }
   }
 
@@ -1094,18 +1093,16 @@ void LayerTreeHost::ApplyScrollAndScale(ScrollAndScaleSet* info) {
       !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(
@@ -1114,7 +1111,7 @@ void LayerTreeHost::ApplyScrollAndScale(ScrollAndScaleSet* info) {
     }
 
     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,