Add assert for permitted_domains.
Add assert for permitted_domains.

Add an assert to protect from user mistakes as the
one that was fixed in https://chromereviews.googleplex.com/93877013/

Unfortunately this code is usually never parsed until it's run
on the production master (I filed a bug a while back for exactly
that: https://code.google.com/p/chromium/issues/detail?id=173336).

BUG=

Review URL: https://codereview.chromium.org/610383002

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@293804 0039d316-1c4b-4281-b951-d872f2087c98

eunchong/build,eunchong/build,eunchong/build,eunchong/build