Move InlineTextBox::paint guts into blink::TextPainter.

This makes it a little clearer than when we have 12-argument functions.

BUG=388946

Review URL: https://codereview.chromium.org/538663002

git-svn-id: svn://svn.chromium.org/blink/trunk@181471 bbb929c8-8fbe-4397-9dbb-9b2b20218538