Add histogram for preload scanner delay.

This shows how much time elapses between when the parser thread
discovers a resource and it's fetched on the main thread.

BUG=None

Review URL: https://chromiumcodereview.appspot.com/22543004

git-svn-id: svn://svn.chromium.org/blink/trunk@155817 bbb929c8-8fbe-4397-9dbb-9b2b20218538