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/cef_frame.h

@@ -40,6 +40,7 @@
 
 #include "include/cef_base.h"
 #include "include/cef_dom.h"
+#include "include/cef_process_message.h"
 #include "include/cef_request.h"
 #include "include/cef_stream.h"
 #include "include/cef_string_visitor.h"
@@ -248,6 +249,16 @@ class CefFrame : public virtual CefBaseRefCounted {
   virtual CefRefPtr<CefURLRequest> CreateURLRequest(
       CefRefPtr<CefRequest> request,
       CefRefPtr<CefURLRequestClient> client) = 0;
+
+  ///
+  // Send a message to the specified |target_process|. Message delivery is not
+  // guaranteed in all cases (for example, if the browser is closing,
+  // navigating, or if the target process crashes). Send an ACK message back
+  // from the target process if confirmation is required.
+  ///
+  /*--cef()--*/
+  virtual void SendProcessMessage(CefProcessId target_process,
+                                  CefRefPtr<CefProcessMessage> message) = 0;
 };
 
 #endif  // CEF_INCLUDE_CEF_FRAME_H_