Call out cross-site data disclosure as a high-severity security bug.

Change-Id: Ide1b92ea746db1bee5c6cc9f12bbbc875fa58341
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1790380
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Charlie Reis <creis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#698638}

docs/security/severity-guidelines.md

@@ -82,8 +82,11 @@ compromised renderer, leading to a sandbox escape
 compromised renderer ([377392](https://crbug.com/377392)).
 * Memory corruption in the browser process that requires specific user
 interaction, such as granting a permission ([455735](https://crbug.com/455735)).
-* Cross-site execution contexts unexpectedly sharing a renderer process despite
-Site Isolation ([863069](https://crbug.com/863069)).
+* Site Isolation bypasses:
+    - Cross-site execution contexts unexpectedly sharing a renderer process
+      ([863069](https://crbug.com/863069), [886976](https://crbug.com/886976)).
+    - Cross-site data disclosure
+      ([917668](https://crbug.com/917668), [927849](https://crbug.com/927849)).
 
 
 ## Medium severity {#TOC-Medium-severity}