DevTools: Preload HTML documents content before fetching resource content.

This is a last step in fixing the issue of HTML document content being unavailable when DevTools is first opened.
This patch adds HTML document content fetching logic as well as fixing all the tests that somehow depend on it.

BUG=354447

Review URL: https://codereview.chromium.org/337783003

git-svn-id: svn://svn.chromium.org/blink/trunk@177538 bbb929c8-8fbe-4397-9dbb-9b2b20218538