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The Chromium IDL parser was updated to support Promise<T> syntax.
Update the IDLs to include types from the spec. Note that the type
isn't actually used for anything so this is a no-op change, for
documentation only.

Review URL: https://codereview.chromium.org/723643002

git-svn-id: svn://svn.chromium.org/blink/trunk@185381 bbb929c8-8fbe-4397-9dbb-9b2b20218538