Update firstrun OWNERS.

Adding skym@ and wenyufu@, who have recently made feature modifications
to the firstrun. Removing yusufo@ who is not currently making changes
in this space and is also an OWNER in a parent folder anyways.

Change-Id: I9343df6eb96a52fa3e381b9859ec8c2198485a63
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2417300Reviewed-by: Wenyu Fu <wenyufu@chromium.org>
Reviewed-by: Yusuf Ozuysal <yusufo@chromium.org>
Commit-Queue: Sky Malice <skym@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809318}

chrome/android/java/src/org/chromium/chrome/browser/firstrun/OWNERS

 bsazonov@chromium.org
 pkotwicz@chromium.org
-yusufo@chromium.org
+skym@chromium.org
+wenyufu@chromium.org