Add initial Chrome runtime support for browser APIs (see issue #2969)

This change adds basic Chrome runtime implementations for CefBrowserContext
and CefBrowserPlatformDelegate. A Chrome browser window with default frame
and styling can now be created using CefBrowserHost::CreateBrowser and some
CefClient callbacks will be triggered via the WebContentsObserver
implementation in CefBrowserHostImpl.

Any additional browser windows created via the Chrome UI will be unmanaged
by CEF. The application message loop will block until all browser windows
have been closed by the user.

libcef/browser/browser_platform_delegate.h

@@ -93,6 +93,10 @@ class CefBrowserPlatformDelegate {
       bool user_gesture,
       bool* was_blocked);
 
+  // Called when the WebContents is destroyed. This will be called before
+  // BrowserDestroyed(). Will only be called a single time per instance.
+  virtual void WebContentsDestroyed(content::WebContents* web_contents);
+
   // See WebContentsDelegate documentation.
   virtual bool ShouldTransferNavigation(bool is_main_frame_navigation);