Move message routing from CefBrowser to CefFrame (see issue #2498).

This change moves the SendProcessMessage method from CefBrowser to CefFrame and
adds CefBrowser parameters to OnProcessMessageReceived and
OnDraggableRegionsChanged.

The internal implementation has changed as follows:
- Frame IDs are now a 64-bit combination of the 32-bit render_process_id and
  render_routing_id values that uniquely identify a RenderFrameHost (RFH).
- CefFrameHostImpl objects are now managed by CefBrowserInfo with life span tied
  to RFH expectations. Specifically, a CefFrameHostImpl object representing a
  sub-frame will be created when a RenderFrame is created in the renderer
  process and detached when the associated RenderFrame is deleted or the
  renderer process in which it runs has died.
- The CefFrameHostImpl object representing the main frame will always be valid
  but the underlying RFH (and associated frame ID) may change over time as a
  result of cross-origin navigations. Despite these changes calling LoadURL on
  the main frame object in the browser process will always navigate as expected.
- Speculative RFHs, which may be created as a result of a cross-origin
  navigation and discarded if that navigation is not committed, are now handled
  correctly (e.g. ignored in most cases until they're committed).
- It is less likely, but still possible, to receive a CefFrame object with an
  invalid frame ID (ID < 0). This can happen in cases where a RFH has not yet
  been created for a sub-frame. For example, when OnBeforeBrowse is called
  before initiating navigation in a previously nonexisting sub-frame.

To test: All tests pass with NetworkService enabled and disabled.

include/capi/cef_drag_handler_capi.h

@@ -33,7 +33,7 @@
 // by hand. See the translator.README.txt file in the tools directory for
 // more information.
 //
-// $hash=22ebb1d811a4e0a834eb115859d797c72a5c4ca3$
+// $hash=5c7eb1cee72dcb8b7657007b3f9db64680ea8c4d$
 //
 
 #ifndef CEF_INCLUDE_CAPI_CEF_DRAG_HANDLER_CAPI_H_
@@ -43,6 +43,7 @@
 #include "include/capi/cef_base_capi.h"
 #include "include/capi/cef_browser_capi.h"
 #include "include/capi/cef_drag_data_capi.h"
+#include "include/capi/cef_frame_capi.h"
 
 #ifdef __cplusplus
 extern "C" {
@@ -79,6 +80,7 @@ typedef struct _cef_drag_handler_t {
   void(CEF_CALLBACK* on_draggable_regions_changed)(
       struct _cef_drag_handler_t* self,
       struct _cef_browser_t* browser,
+      struct _cef_frame_t* frame,
       size_t regionsCount,
       cef_draggable_region_t const* regions);
 } cef_drag_handler_t;