[Sheriff] Disable flaky ExtensionWebRequestApiTest.WebRequestExtraHeaders

Flaky on MacOS: https://ci.chromium.org/p/chromium/builders/ci/Mac10.13%20Tests%20%28dbg%29/17183

TBR=battre@chromium.org

Bug: 1003661
Change-Id: I570c8ff44a394ad5bf62669d73f89d0060310fb6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2050041
Commit-Queue: Kristi Park <kristipark@chromium.org>
Reviewed-by: Kristi Park <kristipark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#740488}

chrome/browser/extensions/api/web_request/web_request_apitest.cc

@@ -611,9 +611,8 @@ IN_PROC_BROWSER_TEST_F(ExtensionWebRequestApiTest,
       << message_;
 }
 
-// Flaky on ChromeOS: https://crbug.com/1003661 and Linux:
-// https://crbug.com/1864717
-#if defined(OS_CHROMEOS) || defined(OS_LINUX)
+// Flaky on multiple platforms: https://crbug.com/1003661
+#if defined(OS_CHROMEOS) || defined(OS_LINUX) || defined(OS_MACOSX)
 #define MAYBE_WebRequestExtraHeaders DISABLED_WebRequestExtraHeaders
 #else
 #define MAYBE_WebRequestExtraHeaders WebRequestExtraHeaders