PNG decoder interlace buffer should be OwnPtr

Make the interlace buffer OwnPtr so it automatically cleans itself
up when the png reader class destructs.

BUG=418113

Review URL: https://codereview.chromium.org/627783002

git-svn-id: svn://svn.chromium.org/blink/trunk@183214 bbb929c8-8fbe-4397-9dbb-9b2b20218538