Mac: Add Views API support (see issue #1749)

The Chrome browser can now be hosted in a Views-based application on Mac
(see issue #2969).

To launch a fully-featured Chrome window using cefsimple:
$ open cefsimple.app --args --enable-chrome-runtime

To launch a minimally-styled Views-hosted window using cefsimple:
$ open cefsimple.app --args --use-views [--enable-chrome-runtime]

To launch a fully-styled Views-hosted window using cefclient:
$ open cefclient.app --args --use-views [--enable-chrome-runtime]

Known issues:
- Some Views unit tests are currently failing on Mac.

libcef/browser/browser_platform_delegate.h

@@ -57,7 +57,7 @@ class Size;
 class Vector2d;
 }  // namespace gfx
 
-#if defined(USE_AURA)
+#if defined(TOOLKIT_VIEWS)
 namespace views {
 class Widget;
 }