Add a unit test for WebCore::GlyphBuffer.

This class is fairly simple, but it's worth taking the rare opportunity
to add a unit test to Blink.

Review URL: https://codereview.chromium.org/343303003

git-svn-id: svn://svn.chromium.org/blink/trunk@177042 bbb929c8-8fbe-4397-9dbb-9b2b20218538