Add doctype declarations when using PageSerializer

Serialized pages were missing their doctypes. This can cause some pages to
rendered incorrectly.

R=abarth@chromium.org
BUG=

Review URL: https://chromiumcodereview.appspot.com/22926011

git-svn-id: svn://svn.chromium.org/blink/trunk@156781 bbb929c8-8fbe-4397-9dbb-9b2b20218538