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/web_contents_dialog_helper.cc

@@ -9,7 +9,7 @@
 #include "chrome/browser/platform_util.h"
 #include "components/web_modal/web_contents_modal_dialog_manager.h"
 
-#if defined(USE_AURA)
+#if defined(TOOLKIT_VIEWS)
 #include "ui/views/widget/widget.h"
 #endif
 
@@ -38,7 +38,7 @@ CefWebContentsDialogHelper::GetWebContentsModalDialogHost() {
 }
 
 gfx::NativeView CefWebContentsDialogHelper::GetHostView() const {
-#if defined(USE_AURA)
+#if defined(TOOLKIT_VIEWS)
   return browser_delegate_->GetWindowWidget()->GetNativeView();
 #else
   NOTIMPLEMENTED();