Validate frame after conversion in chrome.send

BUG=797511
TEST=Manually, see https://crbug.com/797511#c1

Change-Id: Ib1a99db4d7648fb1325eb6d7af4ef111d6dda4cb
Reviewed-on: https://chromium-review.googlesource.com/844076
Commit-Queue: Rob Wu <rob@robwu.nl>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526197}
std::string WebUIExtension::GetVariableValue(const std::string& name) {
  blink::WebLocalFrame* frame;
  RenderFrame* render_frame;
  if (!ShouldRespondToRequest(&frame, &render_frame))
    return std::string();

  return WebUIExtensionData::Get(render_frame->GetRenderView())->GetValue(name);
}

std::string WebUIExtension::GetVariableValue(const std::string& name) {
  blink::WebLocalFrame* frame;
  RenderFrame* render_frame;
  if (!ShouldRespondToRequest(&frame, &render_frame))
    return std::string();

  return WebUIExtensionData::Get(render_frame->GetRenderView())->GetValue(name);
}
