Relocate ShapeResult's GlyphBuffer helpers to ShapeResultBuffer

Follow-up to http://crrev.com/1557743002: the GlyphBuffer builders now operate
on ShapeResultBuffers, so it makes sense to implement as methods.

Code shuffling - no functional changes.

R=eae@chromium.org,drott@chromium.org

Review URL: https://codereview.chromium.org/1561633002

Cr-Commit-Position: refs/heads/master@{#367658}
6 files changed