Link to the right public doc for extension-content-script-fetches.

Before this CL, a comment in
//extensions/browser/url_loader_factory_manager.cc used to incorrectly
link to an internal doc with a draft of
https://www.chromium.org/Home/chromium-security/extension-content-script-fetches
(this comment landed back when we were still considering just making the
doc public, rather than moving the content under
https://www.chromium.org/Home/chromium-security).

Bug: 846346
Change-Id: I55a09bc55523bb0f59d21133402da8a53dd94625
Reviewed-on: https://chromium-review.googlesource.com/c/1487351Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Charlie Reis <creis@chromium.org>
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635191}

extensions/browser/url_loader_factory_manager.cc

@@ -57,7 +57,7 @@ enum class FactoryUser {
 // point they can be removed from the allowlist.
 //
 // Migration plan for extension developers is described at
-// https://docs.google.com/document/d/1wOFTUmwM4NsDfjY4K6aXAbIyH7WJwLFPPIb5KAa1YZ8
+// https://chromium.org/Home/chromium-security/extension-content-script-fetches
 const char* kHardcodedPartOfCorbAllowlist[] = {
     "0149C10F1124F1ED6ACAD85C45E87A76A9DDC667",
     "072D729E856B1F2C9894AEEC3A5DF65E519D6BEE",