Coalesce writes in SVGPathByteStreamBuilder

Add a helper class CoalescingBuffer, that collects the data for a segment
into a local (stack) buffer, and then writes the entire stack buffer in
one go to the SVGPathByteStream.

This reduces codesize (on GCC 4.8) by roughly 50% (closePath being the
only method that grows).

Review URL: https://codereview.chromium.org/149913004

git-svn-id: svn://svn.chromium.org/blink/trunk@166523 bbb929c8-8fbe-4397-9dbb-9b2b20218538