Remove expectation for fixed fast/encoding/api/streaming-decode.html

Flakiness dashboard says Blink r159607 resolved this as expected.

TBR=eae@chromium.org
BUG=305476

Review URL: https://codereview.chromium.org/27354002

git-svn-id: svn://svn.chromium.org/blink/trunk@159677 bbb929c8-8fbe-4397-9dbb-9b2b20218538