DevTools: Load document (html) content from disk cache in page agent enabling.

BUG=354447
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/267393003

git-svn-id: svn://svn.chromium.org/blink/trunk@174557 bbb929c8-8fbe-4397-9dbb-9b2b20218538