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.

include/capi/cef_frame_handler_capi.h

@@ -33,7 +33,7 @@
 // by hand. See the translator.README.txt file in the tools directory for
 // more information.
 //
-// $hash=503984bf98aa52ff67ce52f26a560bbb1d4439bc$
+// $hash=f6be5f7509ee3ccfe16f226470897223cc131014$
 //
 
 #ifndef CEF_INCLUDE_CAPI_CEF_FRAME_HANDLER_CAPI_H_
@@ -147,11 +147,14 @@ typedef struct _cef_frame_handler_t {
 
   ///
   // Called when a frame can begin routing commands to/from the associated
-  // renderer process. Any commands that were queued have now been dispatched.
+  // renderer process. |reattached| will be true (1) if the frame was re-
+  // attached after exiting the BackForwardCache. Any commands that were queued
+  // have now been dispatched.
   ///
   void(CEF_CALLBACK* on_frame_attached)(struct _cef_frame_handler_t* self,
                                         struct _cef_browser_t* browser,
-                                        struct _cef_frame_t* frame);
+                                        struct _cef_frame_t* frame,
+                                        int reattached);
 
   ///
   // Called when a frame loses its connection to the renderer process and will