Avoid a crash with a hacky null check in GetURLForDebugTrace().

This is called when there's no frame for the WebWidget, which is bad.
Since this is a problem in M71, here's something small for an easy
merge. I have a better fix for at least one case where this can happen
in a follow-up, in which I'll revert this hack.

R=enne@chromium.org

Change-Id: Icbffcad6a0fe74bf1ae47e1d51c65aba844ccc5e
Bug: 896836
Reviewed-on: https://chromium-review.googlesource.com/c/1292711Reviewed-by: enne <enne@chromium.org>
Commit-Queue: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601339}

third_party/blink/renderer/core/exported/web_view_impl.cc

@@ -2860,6 +2860,10 @@ void WebViewImpl::ShowContextMenu(WebMenuSourceType source_type) {
 
 WebURL WebViewImpl::GetURLForDebugTrace() {
   WebFrame* main_frame = MainFrame();
+  // TODO(crbug.com/896836): Avoid a crash in minimal way for merge. But we'll
+  // avoid it properly in a followup.
+  if (!main_frame)
+    return {};
   if (main_frame->IsWebLocalFrame())
     return main_frame->ToWebLocalFrame()->GetDocument().Url();
   return {};