DevTools: Support indexing for search across file implementation on front-end.

BUG=263313
R=pfeldman@chromium.org
TBR=darin@chromium.org

Review URL: https://chromiumcodereview.appspot.com/20029005

git-svn-id: svn://svn.chromium.org/blink/trunk@155474 bbb929c8-8fbe-4397-9dbb-9b2b20218538