Use the new hook to add the diff-mode selector to the area directly before
the diff table.
Also toggles the new inline-diff legend, when the initial diff-type is 'inline'.
Depends-On: I2a3c67bcfa47313dee597e602a62073e4e298cd2
Bug: T324759
Change-Id: I1584a84b3caea9eb142afba976c6ff47650c3832