Use memcpy in SVGPathByteStreamSource::readType

This avoids having the compiler beating around the bush trying to
recognize the memcpy, and results in smaller code (roughly 50% smaller
measured for GCC 4.8). All at the affordable price of upsetting a few
language purists.

Review URL: https://codereview.chromium.org/133163007

git-svn-id: svn://svn.chromium.org/blink/trunk@166516 bbb929c8-8fbe-4397-9dbb-9b2b20218538