[Fetch] Add a deprecation warning for Request.context

LayoutTests/http/tests/fetch/chromium/error-messages-expected.txt

Index: LayoutTests/http/tests/fetch/chromium/error-messages-expected.txt
diff --git a/LayoutTests/http/tests/fetch/chromium/error-messages-expected.txt b/LayoutTests/http/tests/fetch/chromium/error-messages-expected.txt
index de1634e0cdfae8641056deb35471b6d1c8ca8f35..80fef246b69f00b3db02e39d19a3f30a1dcf858b 100644
--- a/LayoutTests/http/tests/fetch/chromium/error-messages-expected.txt
+++ b/LayoutTests/http/tests/fetch/chromium/error-messages-expected.txt
@@ -1,5 +1,6 @@
 CONSOLE ERROR: Fetch API cannot load http://localhost:8000/fetch/resources/doctype.html. No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'http://127.0.0.1:8000' is therefore not allowed access.
 CONSOLE ERROR: Fetch API cannot load http://localhost:8000/fetch/resources/redirect-loop.php?Count=100&ACAOrigin=*. Redirect failed.
+CONSOLE WARNING: Request.context is deprecated and will be removed in M46 (see: https://www.chromestatus.com/feature/5534702526005248).
 This is a testharness.js-based test.
 PASS fetch() with 200 should not output error messages 
 PASS fetch() with 404 should not output error messages 
@@ -7,5 +8,6 @@ PASS net::ERR_TOO_MANY_REDIRECTS should not output error messages
 PASS net::ERR_INVALID_CHUNKED_ENCODING should not output error messages 
 PASS A didFailAccessControlCheck() error should output an error message 
 PASS A didFailRedirectCheck() error should output an error message 
+PASS A deprecation warning should be output for Request.context 
 Harness: the test ran to completion.