Improved advice on handling Safari/WKWebView bugs.

Bug: None
Change-Id: I5d24ad95d415a665e5317a803f3b05efff603429
TBR: rsesek
Reviewed-on: https://chromium-review.googlesource.com/996578Reviewed-by: Chris Palmer <palmer@chromium.org>
Commit-Queue: Chris Palmer <palmer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548262}

docs/security/sheriff.md

@@ -206,6 +206,8 @@ necessary bugs.
 * Assign severity, impact, milestone, and component labels
 * CC Apple friends (if you don't know who they are, ping awhalley@)
 * Label **ExternalDependency**
+* File the bug at [bugs.webkit.org](https://bugs.webkit.org) or with
+  product-security@apple.com.
 
 ### Find An Owner To Fix The Bug