Fix draggable region update with BackForwardCache enabled (see issue #2421)

When BackForwardCache is enabled and the user navigates the main frame
back/forward a new RFH may be created for an existing main frame GlobalId value
and CefFrameHostImpl (e.g. an object that was previously Detach()ed after main
frame navigation called SetMainFrame, but for which RenderFrameDeleted was not
subsequently called due to insertion in the BackForwardCache). In this case we
can re-attach the new RFH to the existing main frame CefFrameHostImpl in
RenderFrameHostStateChanged and resume processing of messages.

Swapping back/forward to an existing (already loaded) renderer does not trigger
new notifications for draggable regions (e.g. RenderFrameObserver::
DraggableRegionsChanged is not called by default). We therefore explicitly
request an update of draggable regions by sending the DidStopLoading message to
the renderer.

A new |reattached| parameter is added to CefFrameHandler::OnFrameAttached to
support identification of BackForwardCache usage by the client.

To test with unit tests:
Run `ceftests --gtest_filter=DraggableRegionsTest.DraggableRegionsCrossOrigin
              --enable-features=BackForwardCache`

To test manually:
1. Run `cefclient --enable-features=BackForwardCache --use-views
   --url=http://tests/draggable`, note that draggable regions work.
2. Load https://www.google.com via the address bar, note that draggable regions
   are removed.
3. Go back to http://tests/draggable, note that draggable regions work.
4. Go forward to https://www.google.com, note that draggable regions are
   removed.

libcef/renderer/frame_impl.cc

@@ -335,6 +335,12 @@ void CefFrameImpl::OnDidFinishLoad() {
 }
 
 void CefFrameImpl::OnDraggableRegionsChanged() {
+  // Match the behavior in ChromeRenderFrameObserver::DraggableRegionsChanged.
+  // Only the main frame is allowed to control draggable regions, to avoid other
+  // frames manipulate the regions in the browser process.
+  if (frame_->Parent() != nullptr)
+    return;
+
   blink::WebVector<blink::WebDraggableRegion> webregions =
       frame_->GetDocument().DraggableRegions();
   std::vector<cef::mojom::DraggableRegionEntryPtr> regions;
@@ -508,6 +514,11 @@ void CefFrameImpl::DidStopLoading() {
   // the same browser then the other occurrences will be discarded in
   // OnLoadingStateChange.
   browser_->OnLoadingStateChange(false);
+
+  // Refresh draggable regions. Otherwise, we may not receive updated regions
+  // after navigation because LocalFrameView::UpdateDocumentAnnotatedRegion
+  // lacks sufficient context.
+  OnDraggableRegionsChanged();
 }
 
 void CefFrameImpl::MoveOrResizeStarted() {