sheriff: disable one page capture API test

Specifically ExtensionPageCaptureApiTest.SaveAsMHTMLWithoutFileAccess,
which is flaky everywhere.

Bug: 1156323
Change-Id: Ie47a1e830df1ae145cb593f5a4c6516c4dcf2cab
Tbr: lazyboy@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2577588Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#834469}

chrome/browser/extensions/api/page_capture/page_capture_apitest.cc

@@ -135,8 +135,9 @@ INSTANTIATE_TEST_SUITE_P(ServiceWorker,
                          ExtensionPageCaptureApiTest,
                          ::testing::Values(ContextType::kServiceWorker));
 
+// Flaky on all platforms: https://crbug.com/1156323
 IN_PROC_BROWSER_TEST_P(ExtensionPageCaptureApiTest,
-                       SaveAsMHTMLWithoutFileAccess) {
+                       DISABLED_SaveAsMHTMLWithoutFileAccess) {
   ASSERT_TRUE(StartEmbeddedTestServer());
   PageCaptureSaveAsMHTMLDelegate delegate;
   ASSERT_TRUE(RunTestWithFlagsAndArg("page_capture",