Add support for shared texture and external BeginFrame in OSR mode (issue #1006)

- Add CefWindowInfo::shared_texture_enabled and
  CefRenderHandler::OnAcceleratedPaint for shared texture support. Currently
  only supported on Windows (D3D11).
- Add CefWindowInfo::external_begin_frame_enabled and
  CefBrowserHost::SendExternalBeginFrame for external begin frame support.

libcef/browser/native/browser_platform_delegate_native_win.h

@@ -14,7 +14,9 @@ class CefBrowserPlatformDelegateNativeWin
     : public CefBrowserPlatformDelegateNative {
  public:
   CefBrowserPlatformDelegateNativeWin(const CefWindowInfo& window_info,
-                                      SkColor background_color);
+                                      SkColor background_color,
+                                      bool use_shared_texture,
+                                      bool use_external_begin_frame);
 
   // CefBrowserPlatformDelegate methods:
   void BrowserDestroyed(CefBrowserHostImpl* browser) override;