Deprecate HTMLHtmlElement.manifest

Intent to Deprecate:
https://groups.google.com/a/chromium.org/d/msg/blink-dev/DZecJyapzjM/zv25Xu1t5_oJ

BUG=none

Review URL: https://codereview.chromium.org/236813004

git-svn-id: svn://svn.chromium.org/blink/trunk@171503 bbb929c8-8fbe-4397-9dbb-9b2b20218538