Create 'type' annotation for 'const' variables

From https://developers.google.com/closure/compiler/docs/js-for-compiler:
  "The type declaration and additional comment are optional. If you
  provide a type declaration, put the declaration on a separate line."
Add 'const' annotation to ol.MapBrowserEvent.browserEvent
Merge pull request #1491 from fredj/const-annotation

Create 'type' annotation for 'const' variables
Merge remote-tracking branch 'openlayers/master' into vector-api