Rename region line offset methods

Because of the large number of overloads on the line offset methods,
it is very hard to read code using them, which also makes it harder to
move them out of RenderBlock. This patch renames the methods that take
a region as an argument to make it clearer when looking at the code
how they differ from the ones that don't.

Review URL: https://codereview.chromium.org/59033011

git-svn-id: svn://svn.chromium.org/blink/trunk@161490 bbb929c8-8fbe-4397-9dbb-9b2b20218538