Remove content codings related FIXME comments.

According to https://github.com/slightlyoff/ServiceWorker/issues/366 ability to control content codings has been removed from fetch()

https://github.com/whatwg/fetch/commit/43f8a7b90f07325a58cc31c33129522443c8b843


BUG=400616

Review URL: https://codereview.chromium.org/438183002

git-svn-id: svn://svn.chromium.org/blink/trunk@179573 bbb929c8-8fbe-4397-9dbb-9b2b20218538