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/browser/net/url_request_manager.cc

@@ -9,6 +9,7 @@
 #include "include/cef_scheme.h"
 #include "libcef/browser/browser_context.h"
 #include "libcef/browser/browser_host_impl.h"
+#include "libcef/browser/net/net_util.h"
 #include "libcef/browser/net/resource_request_job.h"
 #include "libcef/browser/net/scheme_handler.h"
 #include "libcef/browser/thread_util.h"
@@ -247,10 +248,11 @@ net::URLRequestJob* CefURLRequestManager::GetRequestJob(
       GetHandlerFactory(request, scheme);
   if (factory.get()) {
     CefRefPtr<CefBrowserHostImpl> browser =
-        CefBrowserHostImpl::GetBrowserForRequest(request);
+        net_util::GetBrowserForRequest(request);
     CefRefPtr<CefFrame> frame;
-    if (browser.get())
-      frame = browser->GetFrameForRequest(request);
+    if (browser.get()) {
+      frame = net_util::GetFrameForRequest(browser->browser_info(), request);
+    }
 
     // Populate the request data.
     CefRefPtr<CefRequestImpl> requestPtr(new CefRequestImpl());