Fix incorrect padding size with NTP.

ContentLayer::ComputeSize() could return the wrong value when
can_use_live_layer is false because the live layer would be hidden by
detaching it from the parent layer.  That's not observable by
the early-returns in GetDrawsContentLeaf, so a full live layer size
could be returned, resulting in insufficient padding.  Switching to
HideLayerAndSubtree instead makes it observed by one early-return.

BUG=696977,699478

Review-Url: https://codereview.chromium.org/2739103004
Cr-Commit-Position: refs/heads/master@{#455999}

chrome/browser/android/compositor/layer/content_layer.cc

@@ -64,9 +64,9 @@ void ContentLayer::SetProperties(int id,
                                  float saturation,
                                  bool should_clip,
                                  const gfx::Rect& clip) {
-  scoped_refptr<cc::Layer> live_layer;
-  if (can_use_live_layer)
-    live_layer = tab_content_manager_->GetLiveLayer(id);
+  scoped_refptr<cc::Layer> live_layer = tab_content_manager_->GetLiveLayer(id);
+  if (live_layer)
+    live_layer->SetHideLayerAndSubtree(!can_use_live_layer);
   bool live_layer_draws = GetDrawsContentLeaf(live_layer);
 
   scoped_refptr<ThumbnailLayer> static_layer =