SideBySide2: Don't open draft on right click in gutter

If the user right clicks on a line number in the gutter, allow
the browser default context menu to popup and do not create a new
draft comment.

Change-Id: I55352b805db5375134f3218c46c5289959a9891a
Merge "SideBySide2: Don't open draft on right click in gutter"