Revert "Do not add sub-resources for data url in MHTML serialization"

This reverts commit 52ce8f4c0454a9eeb0763de11e79d32ece5b634a (r154642).

Consistently fails the new WebPageNewSerializeTest.TestMHTMLEncodingWithDataURL
test on all platforms:
http://test-results.appspot.com/dashboards/flakiness_dashboard.html#group=%40ToT%20Blink&testType=webkit_unit_tests&showExpectations=true&tests=WebPageNewSerializeTest.TestMHTMLEncodingWithDataURL

TBR=qsr@chromium.org
BUG=

Review URL: https://codereview.chromium.org/19619005

git-svn-id: svn://svn.chromium.org/blink/trunk@154655 bbb929c8-8fbe-4397-9dbb-9b2b20218538