Expose latin1 methods in WebString

Needed to plumb header values correctly between Chromium and Blink.

Also temporarily mark a test as flaky in advance of breaking it for a
WebURLLoaderImpl change. When that change lands, the test will be fixed and
switched back.

BUG=276769

Review URL: https://chromiumcodereview.appspot.com/23456013

git-svn-id: svn://svn.chromium.org/blink/trunk@157238 bbb929c8-8fbe-4397-9dbb-9b2b20218538