Increase font size in Gerrit diff messages

Raising it by 1pt to 9pt to match the font size in commit
messages set in 07e32eedf and, to quote the submitter,
'to make it easier to the eyes of old people.'

(This is a local change, but might as well be pushed upstream
if it's a good idea.)

Bug: 40941
Change-Id: I771580d2a9252369b7b38e2a005ccf9248c1f814
Merge "Increase font size in Gerrit diff messages" into production