Add logging for missing resource.

Html viewer is hitting a NOTREACHED due to a missing resource.
Added logging for the resource name so that it can be tracked.

Review URL: https://codereview.chromium.org/1007913002

Cr-Commit-Position: refs/heads/master@{#320759}

mojo/services/html_viewer/blink_platform_impl.cc

@@ -159,7 +159,7 @@ blink::WebData BlinkPlatformImpl::loadResource(const char* resource) {
       return blink::WebData(data, length);
     }
   }
-  NOTREACHED() << "Requested resource is unavailable!";
+  NOTREACHED() << "Requested resource is unavailable: " << resource;
   return blink::WebData();
 }