DevTools: fixed typos in Layers3DView comment.

TBR=aandrey,pfeldman
BUG=

Review URL: https://codereview.chromium.org/23166005

git-svn-id: svn://svn.chromium.org/blink/trunk@156630 bbb929c8-8fbe-4397-9dbb-9b2b20218538