Add DVLOG of the URL and BackForwardCacheCanStoreDocumentResult.

This helps with casually observing BFCache's behaviour.

Change-Id: I36e4573cca27c53d36427bf72fa767154b0ee296
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1949803Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Commit-Queue: Fergal Daly <fergal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#721352}

content/browser/frame_host/back_forward_cache_impl.cc

@@ -273,6 +273,8 @@ BackForwardCacheCanStoreDocumentResult BackForwardCacheImpl::CanStoreDocument(
 
   CanStoreRenderFrameHost(&result, rfh);
 
+  DVLOG(1) << "CanStoreDocument: " << rfh->GetLastCommittedURL() << " : "
+           << result.ToString();
   return result;
 }