Add font support to PageSerializer

Previously the PageSerializer (used to serialize pages before saving them
as e.g. MHTML files) didn't have support for web fonts.

R=abarth@chromium.org
BUG=275518

Review URL: https://chromiumcodereview.appspot.com/23202008

git-svn-id: svn://svn.chromium.org/blink/trunk@157670 bbb929c8-8fbe-4397-9dbb-9b2b20218538