DevTools: fix CSS parser to property report last parsed chunk

This patch fixes CSSParser which failed to properly report last
parsed chunk.

BUG=329825

Review URL: https://codereview.chromium.org/208593002

git-svn-id: svn://svn.chromium.org/blink/trunk@169836 bbb929c8-8fbe-4397-9dbb-9b2b20218538