Get rid of the *Byte types used for (de)serializing to SVGPathByteStream

Use a single ByteType<T> instead.

Additionally clean up SVGPathByteStream a bit by delegating two more
methods {isEmpty(), append(SVGPathByteStream*)} to the wrapped WTF::Vector.

Review URL: https://codereview.chromium.org/130563014

git-svn-id: svn://svn.chromium.org/blink/trunk@166481 bbb929c8-8fbe-4397-9dbb-9b2b20218538