Clarify when DocumentLoader's FrameLoader/FrameLoaderClient accessors can be used

third_party/WebKit/Source/web/WebDataSourceImpl.cpp

Index: third_party/WebKit/Source/web/WebDataSourceImpl.cpp
diff --git a/third_party/WebKit/Source/web/WebDataSourceImpl.cpp b/third_party/WebKit/Source/web/WebDataSourceImpl.cpp
index be9c29325f7e946c623d6246538c069561c407ae..10f872153156450b9b7a9fc1e1fb0bd04a815644 100644
--- a/third_party/WebKit/Source/web/WebDataSourceImpl.cpp
+++ b/third_party/WebKit/Source/web/WebDataSourceImpl.cpp
@@ -45,6 +45,8 @@ WebDataSourceImpl* WebDataSourceImpl::create(
     const ResourceRequest& request,
     const SubstituteData& data,
     ClientRedirectPolicy clientRedirectPolicy) {
+  DCHECK(frame);
+
   return new WebDataSourceImpl(frame, request, data, clientRedirectPolicy);
 }