Improve assistive touch scroll support

Currently Brave responds largely as expected to AssistiveTouch mouse actions, however I have noticed using scroll wheels (eg on a device like the Swiftpoint ProPoint) when there are scrollable areas rather than the full window (eg https://playground.jsreport.net/w/admin/dI2_fUqZ ), the scrolling action does not seem to apply as expected. Even using touch to try and scroll one of the areas moves the whole window