Added -webkit-tap-highlight-color to exit-off-canvas anchor

When the off canvas menu is open on a mobile Chrome browser, the .exit-off-canvas anchor has the webkit tap highlight color applied to it.  This fixes the issue by making the tap highlight color transparent.

Signed-off-by: Jake Wilson <jake.e.wilson@gmail.com>
Merge pull request #3945 from Jakobud/off-canvas-tap-highlight-color

Added -webkit-tap-highlight-color to exit-off-canvas anchor