Add support for begin frame scheduling and direct rendering when GPU compositing is disabled (issue #1368).

- Always set the browser process VSync rate (frame rate) to CefSettings.windowless_frame_rate.
- When the `enable-begin-frame-scheduling` command-line flag is specified the VSync rate for all processes will be synchronized to CefSettings.windowless_frame_rate. This flag cannot be used in combination with windowed rendering.
- When the `disable-gpu` and `disable-gpu-compositing` command-line flags are specified the CefRenderHandler::OnPaint method will be called directly from the compositor instead of requiring an additional copy for each frame.
- CefRenderHandler::OnPopupSize now passes view coordinates instead of (potentially scaled) pixel coordinates.
- Add OSR unit tests for 2x (HiDPI) pixel scaling.
- Improve CefRenderHandler documentation.

git-svn-id: https://chromiumembedded.googlecode.com/svn/trunk@1960 5089003a-bbd8-11dd-ad1f-f1f9622dbc98

tests/cefclient/osrenderer.h

@@ -27,6 +27,7 @@ class ClientOSRenderer {
   // Forwarded from CefRenderHandler callbacks.
   void OnPopupShow(CefRefPtr<CefBrowser> browser,
                    bool show);
+  // |rect| must be in pixel coordinates.
   void OnPopupSize(CefRefPtr<CefBrowser> browser,
                    const CefRect& rect);
   void OnPaint(CefRefPtr<CefBrowser> browser,