[DevTools] Avoid repetitive construction on RegExp in HeapSnapshot search.

Also removed hasSubstring + other minor refactotings.

Review URL: https://chromiumcodereview.appspot.com/18282009

git-svn-id: svn://svn.chromium.org/blink/trunk@154091 bbb929c8-8fbe-4397-9dbb-9b2b20218538