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.

libcef/common/cef_messages.h

@@ -51,9 +51,6 @@ IPC_STRUCT_BEGIN(Cef_Request_Params)
   // Unique request id to match requests and responses.
   IPC_STRUCT_MEMBER(int, request_id)
 
-  // Unique id of the target frame. -1 if unknown / invalid.
-  IPC_STRUCT_MEMBER(int64_t, frame_id)
-
   // True if the request is user-initiated instead of internal.
   IPC_STRUCT_MEMBER(bool, user_initiated)
 
@@ -112,10 +109,6 @@ IPC_STRUCT_BEGIN(CefMsg_LoadRequest_Params)
   // One of the cef_referrer_policy_t values.
   IPC_STRUCT_MEMBER(int, referrer_policy)
 
-  // Identifies the frame within the RenderView that sent the request.
-  // -1 if unknown / invalid.
-  IPC_STRUCT_MEMBER(int64_t, frame_id)
-
   // Usually the URL of the document in the top-level window, which may be
   // checked by the third-party cookie blocking policy. Leaving it empty may
   // lead to undesired cookie blocking. Third-party cookie blocking can be
@@ -197,20 +190,15 @@ IPC_SYNC_MESSAGE_CONTROL1_1(
     int /* render_frame_routing_id */,
     CefProcessHostMsg_GetNewBrowserInfo_Params /* params*/)
 
-// Sent when a frame is identified for the first time.
-IPC_MESSAGE_ROUTED3(CefHostMsg_FrameIdentified,
-                    int64_t /* frame_id */,
-                    int64_t /* parent_frame_id */,
-                    base::string16 /* frame_name */)
+// Sent by the renderer when the frame can begin receiving messages.
+IPC_MESSAGE_ROUTED0(CefHostMsg_FrameAttached)
 
 // Sent by the renderer when the frame becomes focused.
 IPC_MESSAGE_ROUTED0(CefHostMsg_FrameFocused)
 
 // Sent when a frame has finished loading. Based on ViewHostMsg_DidFinishLoad.
-IPC_MESSAGE_ROUTED4(CefHostMsg_DidFinishLoad,
-                    int64_t /* frame_id */,
+IPC_MESSAGE_ROUTED2(CefHostMsg_DidFinishLoad,
                     GURL /* validated_url */,
-                    bool /* is_main_frame */,
                     int /* http_status_code */)
 
 // Sent when the renderer has a request for the browser. The browser may respond