Improve efficiency of converting SVG path strings to streams.

Building up a vector byte-by-byte causes a lot of vector expansations (and
therefore copies and spamming of the allocator). In this case, we can make a
reasonable estimate of how much space we'll need and trim it at the end.

BUG=335734

Review URL: https://codereview.chromium.org/151933002

git-svn-id: svn://svn.chromium.org/blink/trunk@166266 bbb929c8-8fbe-4397-9dbb-9b2b20218538