Move isEmptyValue and emptyValue to SVGCharacterData

These two helpers have a stronger tie to SVGCharacterData (on which they
operate) than to SVGTextLayoutAttributes - where they are currently
defined.
Move them as described, and also add simple query helpers to make code
using them simpler and more readable.

BUG=607906

Review-Url: https://codereview.chromium.org/1933183002
Cr-Commit-Position: refs/heads/master@{#390649}
4 files changed