OOR-CORS: update webRequest API document on extraHeaders for preflights

Because of performance implication, we decided to require
'extraHeaders' option to intercept CORS preflights.

Bug: 1002884
Change-Id: I00678e265a0488d18bd44451e5b2b1c6c71ad31c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1847607
Commit-Queue: Takashi Toyoshima <toyoshim@chromium.org>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707738}

chrome/common/extensions/docs/templates/intros/webRequest.html

@@ -130,6 +130,15 @@ checks. If you need to deceive the CORS protocol, you also need to specify
 <code>'extraHeaders'</code> for the response modifications.
 </p>
 
+<p>
+<span class="availability">Starting from Chrome 79</span>, the webRequest API
+<strong>does not</strong> intercept CORS preflight requests and responses by
+default. A CORS preflight for a request URL is visible to an extension if there
+is a listener with <code>'extraHeaders'</code> specified in
+<code>opt_extraInfoSpec</code> for the request URL. <code>onBeforeRequest</code>
+can also take <code>'extraHeaders'</code> from Chrome 79.
+</p>
+
 <p>
 <span class="availability">Starting from Chrome 79</span>, the following request
 header is <strong>not provided</strong> and cannot be modified or removed