Update 3dSortingContextId on content_layer_ when context changes

A previous re-factoring (https://crrev.com/e337262e101e86c301d78ffa4337311f7d2d9110)
accidentally changed a call on content_layer_ to a call on CcLayer(). Fix it.

R=chrishtr@chromium.org
BUG=935002

Change-Id: Ia035921e28d9ad1c0a5518fb97110d20c1434b5f
Reviewed-on: https://chromium-review.googlesource.com/c/1485123
Commit-Queue: Stephen Chenney <schenney@chromium.org>
Commit-Queue: Philip Rogers <pdr@chromium.org>
Auto-Submit: Stephen Chenney <schenney@chromium.org>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634977}

third_party/blink/renderer/platform/graphics/graphics_layer.cc

@@ -705,7 +705,7 @@ void GraphicsLayer::SetRenderingContext(int context) {
   CcLayer()->Set3dSortingContextId(context);
 
   if (contents_layer_)
-    CcLayer()->Set3dSortingContextId(rendering_context3d_);
+    contents_layer_->Set3dSortingContextId(rendering_context3d_);
 }
 
 bool GraphicsLayer::MasksToBounds() const {