Move fetch-related predicates to core/fetch.

Currently core/fetch, core/xml and modules/serviceworkers have predicates
defined in http://fetch.spec.whatwg.org/ such as "simple method".
This CL moves these predicates to core/fetch/FetchUtils.h and
renames some predicates.

BUG=NONE

Review URL: https://codereview.chromium.org/379113002

git-svn-id: svn://svn.chromium.org/blink/trunk@179692 bbb929c8-8fbe-4397-9dbb-9b2b20218538