Fix nullptr deref in maybeRenderFallbackContent()

third_party/WebKit/Source/web/WebLocalFrameImpl.cpp

Index: third_party/WebKit/Source/web/WebLocalFrameImpl.cpp
diff --git a/third_party/WebKit/Source/web/WebLocalFrameImpl.cpp b/third_party/WebKit/Source/web/WebLocalFrameImpl.cpp
index 31776316c9030221eb0e464a4577caf644f0f50b..c30eca8ad06b7506f5784fd044d305ce320c669c 100644
--- a/third_party/WebKit/Source/web/WebLocalFrameImpl.cpp
+++ b/third_party/WebKit/Source/web/WebLocalFrameImpl.cpp
@@ -2115,8 +2115,11 @@ bool WebLocalFrameImpl::MaybeRenderFallbackContent(
   if (!GetFrame()->Owner() || !GetFrame()->Owner()->CanRenderFallbackContent())
     return false;
 
-  DCHECK(GetFrame()->Loader().ProvisionalDocumentLoader());
-  GetFrame()->Loader().ProvisionalDocumentLoader()->LoadFailed(error);
+  // provisionalDocumentLoader() can be null if a navigation started and
+  // completed (e.g. about:blank) while waiting for the navigation that wants
+  // to show fallback content.
+  if (GetFrame()->Loader().ProvisionalDocumentLoader())
+    GetFrame()->Loader().ProvisionalDocumentLoader()->LoadFailed(error);
   return true;
 }