Add new CanZoom/Zoom API (fixes #3284)

Add a simpler CanZoom/Zoom API as an alternative to the more error-prone
SetZoomLevel/GetZoomLevel API. Both APIs are now implemented for both runtimes.
With the Chrome runtime a zoom notification bubble will be displayed unless
CefBrowserSettings.chrome_zoom_bubble is set to STATE_DISABLED.

To test:
- Run cefclient and select zoom entries from the Tests menu.
- chrome: Run cefclient with `--hide-chrome-bubbles` command-line flag to hide
  the zoom notification bubble.

libcef/browser/chrome/chrome_browser_host_impl.cc

@@ -179,15 +179,6 @@ CefWindowHandle ChromeBrowserHostImpl::GetOpenerWindowHandle() {
   return kNullWindowHandle;
 }
 
-double ChromeBrowserHostImpl::GetZoomLevel() {
-  NOTIMPLEMENTED();
-  return 0.0;
-}
-
-void ChromeBrowserHostImpl::SetZoomLevel(double zoomLevel) {
-  NOTIMPLEMENTED();
-}
-
 void ChromeBrowserHostImpl::Find(const CefString& searchText,
                                  bool forward,
                                  bool matchCase,