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

third_party/WebKit/Source/web/DevToolsEmulator.cpp

Index: third_party/WebKit/Source/web/DevToolsEmulator.cpp
diff --git a/third_party/WebKit/Source/web/DevToolsEmulator.cpp b/third_party/WebKit/Source/web/DevToolsEmulator.cpp
index 44ed45095d8384e64a4090c0bc2d08b0f127f028..ac941acb27f029b8cb6383f19c5ad9aee03af2be 100644
--- a/third_party/WebKit/Source/web/DevToolsEmulator.cpp
+++ b/third_party/WebKit/Source/web/DevToolsEmulator.cpp
@@ -316,7 +316,14 @@ void DevToolsEmulator::setTouchEventEmulationEnabled(bool enabled)
         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)