Simplify GlyphBuffer::reverse.

Since only the entire buffer is ever reversed, a simpler and more
obvious implementation using WTF::Vector::reverse is possible.

Review URL: https://codereview.chromium.org/348553003

git-svn-id: svn://svn.chromium.org/blink/trunk@176526 bbb929c8-8fbe-4397-9dbb-9b2b20218538