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.

libcef/browser/native/browser_platform_delegate_native_win.cc

@@ -276,8 +276,9 @@ void CefBrowserPlatformDelegateNativeWin::SizeTo(int width, int height) {
   // based on the current style.
   AdjustWindowRectEx(&rect, style, has_menu, ex_style);
 
-  // Size the window.
-  SetWindowPos(window, NULL, 0, 0, rect.right, rect.bottom,
+  // Size the window. The left/top values may be negative.
+  SetWindowPos(window, NULL, 0, 0, rect.right - rect.left,
+               rect.bottom - rect.top,
                SWP_NOZORDER | SWP_NOMOVE | SWP_NOACTIVATE);
 }