Convert legacy IPC messages to Mojo (fixes issue #3123)

This change introduces a few minor CEF API behavior changes:

- A CefProcessMessage object cannot be reused after being passed to
  SendProcessMessage.
- The |extra_info| argument to CefRenderProcessHandler::OnBrowserCreated may
  now be NULL.

Where appropriate, we now utilize the default UTF string encoding format and
shared memory to reduce copies and conversions for the cross-process
transfer of arbitrary-length strings. For example, CefFrame::GetSource/GetText
now involves zero UTF conversions and zero copies in the browser process for
the CefString delivered to CefStringVisitor::Visit().

include/cef_render_process_handler.h

@@ -66,11 +66,11 @@ class CefRenderProcessHandler : public virtual CefBaseRefCounted {
   ///
   // Called after a browser has been created. When browsing cross-origin a new
   // browser will be created before the old browser with the same identifier is
-  // destroyed. |extra_info| is a read-only value originating from
+  // destroyed. |extra_info| is an optional read-only value originating from
   // CefBrowserHost::CreateBrowser(), CefBrowserHost::CreateBrowserSync(),
   // CefLifeSpanHandler::OnBeforePopup() or CefBrowserView::CreateBrowserView().
   ///
-  /*--cef()--*/
+  /*--cef(optional_param=extra_info)--*/
   virtual void OnBrowserCreated(CefRefPtr<CefBrowser> browser,
                                 CefRefPtr<CefDictionaryValue> extra_info) {}