[DevTools] Check that mainFrameImpl() has been initalized before accessing it.

third_party/WebKit/Source/web/DevToolsEmulator.cpp

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "web/DevToolsEmulator.h"
 
 #include "core/frame/FrameHost.h"
 #include "core/frame/FrameView.h"
 #include "core/frame/Settings.h"
 #include "core/page/Page.h"
@@ skipping 298 matching lines @@
         m_originalMaxTouchPoints = m_webViewImpl->page()->settings().maxTouchPoints();
     }
     RuntimeEnabledFeatures::setTouchEnabled(enabled ? true : m_originalTouchEnabled);
     if (!m_originalDeviceSupportsTouch) {
         m_webViewImpl->page()->settings().setDeviceSupportsMouse(enabled ? false : m_originalDeviceSupportsMouse);
         m_webViewImpl->page()->settings().setDeviceSupportsTouch(enabled ? true : m_originalDeviceSupportsTouch);
         // Currently emulation does not provide multiple touch points.
         m_webViewImpl->page()->settings().setMaxTouchPoints(enabled ? 1 : m_originalMaxTouchPoints);
     }
     m_touchEventEmulationEnabled = enabled;
-    m_webViewImpl->mainFrameImpl()->frameView()->layout();
+    // TODO(dgozman): mainFrameImpl() check in this class should be unnecessary.
+    // It is only needed when we reattach and restore InspectorEmulationAgent,
+    // which happens before everything has been setup correctly, and therefore
+    // fails during remote -> local main frame transition.
+    // We should instead route emulation through the WebViewImpl which has
+    // local main frame, and remove InspectorEmulationAgent entirely.

[nasko, 2016/01/26 19:43:07]

WebViewImpl is not guaranteed to have local main frame in a process that renders
subframes.

[dgozman, 2016/01/27 17:54:44]

Ah, right. But if we send the page-level IPC from browser it should go to the
local main frame, shouldn't it? I rephrased the comment.

[nasko, 2016/01/27 17:55:57]

Page-level IPCs in general should go to all processes, so they can all have the
right state. Otherwise your main frame will be in emulation mode, but the
subframes will not be, right?

+    if (m_webViewImpl->mainFrameImpl())
+        m_webViewImpl->mainFrameImpl()->frameView()->layout();
 }
 
 void DevToolsEmulator::setScriptExecutionDisabled(bool scriptExecutionDisabled)
 {
     m_scriptExecutionDisabled = scriptExecutionDisabled;
     m_webViewImpl->page()->settings().setScriptEnabled(m_scriptExecutionDisabled ? false : m_embedderScriptEnabled);
 }
 
 bool DevToolsEmulator::handleInputEvent(const WebInputEvent& inputEvent)
 {
@@ skipping 24 matching lines @@
             m_lastPinchAnchorCss.clear();
             m_lastPinchAnchorDip.clear();
         }
         return true;
     }
 
     return false;
 }
 
 } // namespace blink