When users navigate to the diff view with a line number specified at the
end, depending on their context preference, the line might be in a
shared region that gets collapsed when the diff renders. With this
change, the location specified in the URL is prevented from being
collapsed by marking it as a "key" location.
Bug: Issue 5247
Change-Id: Ifd5827cd922b022cddb1601911a9ecea6a054f35