The unified diff is better than the side-by-side diff on a
mobile browser with limited screen real estate. It has only one
CodeMirror instance so it doesn't need to synchronize scrolling,
which is a pretty expensive JavaScript operation. Also, the user
doesn't have to scroll horizontally / zoom out that much with a
unified diff.
Note that we only default to unified diff if the view is
portrait, to account for the fact that some tablets have
screens as large as 13 inches.
Change-Id: I385fd336b50b1c036eba5760395bfdb04c2915a9
4d2c564685