Make some ScriptValue references const

The functions FontFaceSet::forEach() and Headers::forEach() are called
from the bindings layer, and by having non-const reference arguments, they
somewhat restrict what we can do in the bindings layer.

Since there is no reason for the arguments to be non-const references,
just make them const.

Review URL: https://codereview.chromium.org/478233003

git-svn-id: svn://svn.chromium.org/blink/trunk@180719 bbb929c8-8fbe-4397-9dbb-9b2b20218538