Add OWNERS for ios/chrome/browser/safe_browsing

This adds an OWNERS file to this recently-added directory.

Change-Id: I3ffd7971c195b73a713b518aec4756f6c6d9778a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2096776Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Ali Juma <ajuma@chromium.org>
Cr-Commit-Position: refs/heads/master@{#748687}

ios/chrome/browser/safe_browsing/OWNERS

+ajuma@chromium.org
+kkhorimoto@chromium.org
+
+# TEAM: ios-directory-owners@chromium.org
+# OS: iOS
+# COMPONENT: UI>Browser>Navigation