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.

tests/cefclient/browser/root_window_create.cc

@@ -4,9 +4,7 @@
 
 #include "tests/cefclient/browser/root_window.h"
 
-#if defined(OS_WIN) || defined(OS_LINUX)
 #include "tests/cefclient/browser/root_window_views.h"
-#endif
 
 #if defined(OS_WIN)
 #include "tests/cefclient/browser/root_window_win.h"
@@ -21,11 +19,7 @@ namespace client {
 // static
 scoped_refptr<RootWindow> RootWindow::Create(bool use_views) {
   if (use_views) {
-#if defined(OS_WIN) || defined(OS_LINUX)
     return new RootWindowViews();
-#else
-    LOG(FATAL) << "Views framework is not supported on this platform.";
-#endif
   }
 
 #if defined(OS_WIN)