Add chrome://flags/#force-empty-CORB-and-CORS-allowlist flag.

This CL adds a new chrome://flag as an easy alternative to manually
ensuring that there are no Chrome processes and then launching Chrome
with --force-empty-corb-allowlist cmdline flag.  The new flag is:
chrome://flags/#force-empty-CORB-and-CORS-allowlist

Additionally the CL slightly tweaks the text and metadata for
the already existing chrome://flag that can be used to opt into
or out of the CORS-for-content-scripts changes that are planned
to ship in Chrome 85.  This existing flag is:
chrome://flags/#cors-for-content-scripts

Screenshot of the flags can be found here:
https://drive.google.com/file/d/1xPpHGMmjgMOYDEZH1U02XvAxDHDPMwoc/view

Bug: 920638
Change-Id: I0d2b44dbf58db7dacb3addc67e4be977cd9437de
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2243717
Reviewed-by: Charlie Reis <[email protected]>
Commit-Queue: Ɓukasz Anforowicz <[email protected]>
Cr-Commit-Position: refs/heads/master@{#778603}
diff --git a/chrome/browser/flag_descriptions.h b/chrome/browser/flag_descriptions.h
index e3e77878..64d155d 100644
--- a/chrome/browser/flag_descriptions.h
+++ b/chrome/browser/flag_descriptions.h
@@ -445,6 +445,8 @@
 
 extern const char kCorsForContentScriptsName[];
 extern const char kCorsForContentScriptsDescription[];
+extern const char kForceEmptyCorbAndCorsAllowlistName[];
+extern const char kForceEmptyCorbAndCorsAllowlistDescription[];
 
 extern const char kCrossOriginOpenerPolicyName[];
 extern const char kCrossOriginOpenerPolicyDescription[];