chrome: Add Views API integration (see issue #2969)

The Chrome browser can now be hosted in a Views-based application on Windows
and Linux.

To launch a fully-featured Chrome window using cefsimple:
$ cefsimple --enable-chrome-runtime

To launch a minimally-styled Views-hosted window using cefsimple:
$ cefsimple --enable-chrome-runtime --use-views

To launch a fully-styled Views-hosted window using cefclient:
$ cefclient --enable-chrome-runtime --use-views

Views unit tests also now pass with the Chrome runtime enabled:
$ ceftests --gtest_filter=Views* --enable-chrome-runtime

Known issues:
- Popup browsers cannot be intercepted and reparented.

libcef/browser/browser_platform_delegate.h

@@ -168,7 +168,7 @@ class CefBrowserPlatformDelegate {
   // the client, which may be NULL. May be called on multiple threads.
   virtual CefWindowHandle GetHostWindowHandle() const;
 
-#if defined(USE_AURA)
+#if defined(TOOLKIT_VIEWS)
   // Returns the Widget owner for the browser window. Only used with windowed
   // rendering.
   virtual views::Widget* GetWindowWidget() const;