Update example bug for browser memory corruption in severity guidelines

319125 is a straightforward browser memory corruption bug where an IPC
handler blindly trusts data from the renderer, forgetting that one of
the types of data that can be set is a raw pointer ^_^

Change-Id: I14a37f1882c06edc56e5d4d2f7c1d0444869bec4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1790480
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Max Moroz <mmoroz@chromium.org>
Reviewed-by: Emily Stark <estark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694943}