Fix history crash with null DocumentLoader

Its possible that the frame doesn't have a document loader anymore.
Check document loader like other paths do in the History object.

BUG=1000206

Change-Id: Idc1eda15c194e26ff818855fc1ac5e595ae693a6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1791421Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695151}

third_party/blink/renderer/core/frame/history.cc

@@ -89,7 +89,7 @@ SerializedScriptValue* History::state(ExceptionState& exception_state) {
 }
 
 SerializedScriptValue* History::StateInternal() const {
-  if (!GetFrame())
+  if (!GetFrame() || !GetFrame()->Loader().GetDocumentLoader())
     return nullptr;
 
   if (HistoryItem* history_item =