Make DOM.getChildNodes & DOM.getDocument optionally pierce iframe boundaries

This means you can easily capture all the DOM nodes on a page with iframes with a single command,
previously multiple calls to DOM.getChildNodes were required.

BUG=546953

Review-Url: https://codereview.chromium.org/2373023002
Cr-Commit-Position: refs/heads/master@{#425063}
8 files changed