Remove a significant source of memory allocations during CSS parsing.

This is about +2% on Parser/css-parser-yui.html locally.

Previously, we were allocating and freeing small vector buffers (most commonly
24 or 72 byte allocations on 64-bit), only to immediately throw them away.

Note that uncheckedAppend has an ASSERT that will cleanly catch any future
problems with sizing.


BUG=310918

Review URL: https://codereview.chromium.org/57813005

git-svn-id: svn://svn.chromium.org/blink/trunk@161293 bbb929c8-8fbe-4397-9dbb-9b2b20218538