Update wtf/OWNERS
Update wtf/OWNERS

- Add sigbjornf@ and yutak@
  They contribute to wtf very actively.

- Remove jchaffraix@ and morrita@
  They don't work on Chromium project any longer.

NOTRY=true

Review-Url: https://codereview.chromium.org/1971973002
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#393205}

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