Early out of registerViewportLayerWithCompositor if there's no local MainFrame.

I moved this code out of VisualViewport in r404359 but forgot this one check
and early-out. The crashes started happening right after that so it's likely the
culprit.

BUG=627442

Review-Url: https://codereview.chromium.org/2140263002
Cr-Commit-Position: refs/heads/master@{#405168}

third_party/WebKit/Source/web/WebViewImpl.cpp

@@ -4193,7 +4193,9 @@ bool WebViewImpl::tabsToLinks() const
 void WebViewImpl::registerViewportLayersWithCompositor()
 {
     DCHECK(m_layerTreeView);
-    DCHECK(page()->deprecatedLocalMainFrame());
+
+    if (!page()->mainFrame() || !page()->mainFrame()->isLocalFrame())
+        return;
 
     Document* document = page()->deprecatedLocalMainFrame()->document();