DevTools: wrap the way test access network requests.

I'm going to refactor NetworkPanel. As a result, accessor will change.

In this patch all references to accessor are replaced with a utility
method, so in future I'll need to change in one place.

BUG=

Review URL: https://codereview.chromium.org/348033002

git-svn-id: svn://svn.chromium.org/blink/trunk@176625 bbb929c8-8fbe-4397-9dbb-9b2b20218538