Add Security Critical field to README.chromium.

As per [1], WebRTC qualifies as a security critical Chromium dependency.

[1] - https://chromium.googlesource.com/chromium/src/+/HEAD/docs/adding_to_third_party.md#add-a-readme_chromium

Bug: chromium:362396795
Change-Id: Ic4fc113cc2e3ec1df1c605698e1b3e83111aa14b
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/360740
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Auto-Submit: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#42858}
diff --git a/README.chromium b/README.chromium
index 333e79e..ea1874d 100644
--- a/README.chromium
+++ b/README.chromium
@@ -5,6 +5,7 @@
 License: BSD
 License File: LICENSE
 Shipped: yes
+Security Critical: yes
 
 Description:
 WebRTC provides real time voice and video processing