Add null check when accessing WebLocalFrameImpl::mainFrameImpl().

The main frame might be null, so we need to check that before trying to
dereference it to see if it's a local frame.

BUG=385162

Review URL: https://codereview.chromium.org/334323002

git-svn-id: svn://svn.chromium.org/blink/trunk@176245 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/web/WebViewImpl.cpp

@@ -442,7 +442,7 @@ WebViewImpl::~WebViewImpl()
 
 WebLocalFrameImpl* WebViewImpl::mainFrameImpl()
 {
-    return m_page && m_page->mainFrame()->isLocalFrame() ? WebLocalFrameImpl::fromFrame(m_page->deprecatedLocalMainFrame()) : 0;
+    return m_page && m_page->mainFrame() && m_page->mainFrame()->isLocalFrame() ? WebLocalFrameImpl::fromFrame(m_page->deprecatedLocalMainFrame()) : 0;
 }
 
 bool WebViewImpl::tabKeyCyclesThroughElements() const