Add support for loading extensions (issue #1947)

- Add CefRequestContext::LoadExtension, CefExtension, CefExtensionHandler and
  related methods/interfaces.
- Add chrome://extensions-support that lists supported Chrome APIs.
- Add CefBrowserHost::SetAutoResizeEnabled and CefDisplayHandler::OnAutoResize
  to support browser resize based on preferred web contents size.
- views: Add support for custom CefMenuButton popups.
- cefclient: Run with `--load-extension=set_page_color` command-line flag for
  an extension loading example. Add `--use-views` on Windows and Linux for an
  even better example.

tests/cefclient/browser/browser_window.h

@@ -37,6 +37,9 @@ class BrowserWindow : public ClientHandler::Delegate {
     // Set fullscreen mode.
     virtual void OnSetFullscreen(bool fullscreen) = 0;
 
+    // Auto-resize contents.
+    virtual void OnAutoResize(const CefSize& new_size) = 0;
+
     // Set the loading state.
     virtual void OnSetLoadingState(bool isLoading,
                                    bool canGoBack,
@@ -117,6 +120,7 @@ class BrowserWindow : public ClientHandler::Delegate {
   void OnSetAddress(const std::string& url) OVERRIDE;
   void OnSetTitle(const std::string& title) OVERRIDE;
   void OnSetFullscreen(bool fullscreen) OVERRIDE;
+  void OnAutoResize(const CefSize& new_size) OVERRIDE;
   void OnSetLoadingState(bool isLoading,
                          bool canGoBack,
                          bool canGoForward) OVERRIDE;