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.cc

@@ -112,7 +112,7 @@ CefWindowHandle CefBrowserPlatformDelegate::GetHostWindowHandle() const {
   return kNullWindowHandle;
 }
 
-#if defined(USE_AURA)
+#if defined(TOOLKIT_VIEWS)
 views::Widget* CefBrowserPlatformDelegate::GetWindowWidget() const {
   NOTREACHED();
   return nullptr;
@@ -122,7 +122,7 @@ CefRefPtr<CefBrowserView> CefBrowserPlatformDelegate::GetBrowserView() const {
   NOTREACHED();
   return nullptr;
 }
-#endif
+#endif  // defined(TOOLKIT_VIEWS)
 
 void CefBrowserPlatformDelegate::PopupWebContentsCreated(
     const CefBrowserSettings& settings,