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We currently decode them as UTF-8, but they should be interpreted as Latin-1.
(XHR specs the relevant parameters as WebIDL ByteString.) For response headers,
this matches Firefox, IE, Opera 12, and Safari.  For status text, this matches
Firefox nightly, IE, Opera 12, and Opera. For request headers, this matches
Firefox nightly, Opera 12, and Safari.

Preparatory Blink CL disabled a layout test:
  https://src.chromium.org/viewvc/blink?revision=157238&view=revision

This CL will be followed by a Blink CL to fix and re-enable the test, throw
exception on non-Latin-1 setRequestHeader argument, and add a new layout test
to verify this behavior.

BUG=276769

Review URL: https://chromiumcodereview.appspot.com/23553007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@222127 0039d316-1c4b-4281-b951-d872f2087c98