Add a renderName to RenderTextFragment.

Currently any RenderTextFragment will show as RenderText in the dump'd tree.
This makes things confusing when debugging as you don't know which object
you're dealing with.

This CL adds a renderName() method to RenderTextFragment to make this more
clear.

Review URL: https://codereview.chromium.org/575833002

git-svn-id: svn://svn.chromium.org/blink/trunk@182114 bbb929c8-8fbe-4397-9dbb-9b2b20218538