Add new user script injection point "document_idle" and make it the default.

Semantically, document-idle means "when the DOM is ready and layout has been idle for awhile", or more loosely, "as soon as we get around to it".

Right now this uses a simple heuristic. It injects scripts 200ms after DOMContentLoaded, or immediately after onload, whichever happens first.

BUG=26126
TEST=Manual. Extensions with content scripts should work.

Review URL: http://codereview.chromium.org/339064

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@30784 0039d316-1c4b-4281-b951-d872f2087c98