Add promise type to EME idl files

Now that the Blink IDL supports the Promise<T> syntax, update
the EME idl files with the promise type.

BUG=358271
TEST=existing EME tests pass

Review URL: https://codereview.chromium.org/694773004

git-svn-id: svn://svn.chromium.org/blink/trunk@185152 bbb929c8-8fbe-4397-9dbb-9b2b20218538