Add null check of the return value of Document::frame() in DTL::reportResponseReceived().

third_party/WebKit/Source/core/loader/DocumentThreadableLoader.cpp

Index: third_party/WebKit/Source/core/loader/DocumentThreadableLoader.cpp
diff --git a/third_party/WebKit/Source/core/loader/DocumentThreadableLoader.cpp b/third_party/WebKit/Source/core/loader/DocumentThreadableLoader.cpp
index 900edce3865cea7e831e3c73c118e28d94873ff5..a22525327dccedc80a0e4cc8b43220a0ea4667db 100644
--- a/third_party/WebKit/Source/core/loader/DocumentThreadableLoader.cpp
+++ b/third_party/WebKit/Source/core/loader/DocumentThreadableLoader.cpp
@@ -570,6 +570,11 @@ void DocumentThreadableLoader::handlePreflightResponse(const ResourceResponse& r
 void DocumentThreadableLoader::reportResponseReceived(unsigned long identifier, const ResourceResponse& response)
 {
     LocalFrame* frame = document().frame();
+    // We are seeing crashes caused by nullptr (crbug.com/578849). But the frame
+    // must be set here. TODO(horo): Find the root cause of the unset frame.
+    ASSERT(frame);
+    if (!frame)
+        return;
     DocumentLoader* loader = frame->loader().documentLoader();
     TRACE_EVENT_INSTANT1("devtools.timeline", "ResourceReceiveResponse", TRACE_EVENT_SCOPE_THREAD, "data", InspectorReceiveResponseEvent::data(identifier, frame, response));
     InspectorInstrumentation::didReceiveResourceResponse(frame, identifier, loader, response, resource() ? resource()->loader() : 0);