Save three bytes per CStringBuffer object.

sizeof(CStringBuffer) was rounded up to 8 on account of struct size and
alignment rules. This is clearly not what was intended.

Also tidy up includes.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/22734002

git-svn-id: svn://svn.chromium.org/blink/trunk@155849 bbb929c8-8fbe-4397-9dbb-9b2b20218538