Handle all non-menu-defined shortcuts appropriately

[sardination]

Bug Description
So far, this has only been a problem for zooming in. The menu shortcut for zooming in is CMD +, but the non-menu-defined shortcut CMD = also works. For some reason, Chromium handles menu-defined and non-menu-defined shortcuts differently. To get around this for zooming in, we implemented the changes seen here: https://github.com/whisthq/brave-core/pull/518.

How to Reproduce
Try a non-menu defined shortcut that should be handled client-side and see how it actually gets handled server-side instead.

Potential Implementation(s)
Figure out what pathway these non-menu-defined shortcuts take and have a universal handler for them that makes sure that the appropriate shortcuts are handled server and client-side.