Add felt@ to Safe Browsing OWNERS file for blocking page changes

BUG=
R=nparker@chromium.org

Review-Url: https://codereview.chromium.org/2895213004
Cr-Commit-Position: refs/heads/master@{#473926}

chrome/browser/safe_browsing/OWNERS

 mattm@chromium.org
 nparker@chromium.org
-
-# For download-protection- and reporting-related changes:
 jialiul@chromium.org
+
+# For interstitial-related changes:
+per-file safe_browsing_blocking_page*=felt@chromium.org