DevTools: [CSS] cache lineEndings in InspectorStyleSheet

lineEndings are used for creation of source range in the protocol
CSS domain. This significantly speeds up all CSS domain protocol methods.

BUG=423823
R=vsevik, pfeldman

Review URL: https://codereview.chromium.org/666883002

git-svn-id: svn://svn.chromium.org/blink/trunk@183981 bbb929c8-8fbe-4397-9dbb-9b2b20218538