[iOS] Marking WKWebViewEarlyPageScriptAccessibility test as flaky

ChromeWebClientTest.WKWebViewEarlyPageScriptAccessibility is flaky.

See:
https://logs.chromium.org/logs/chrome/buildbucket/cr-buildbucket.appspot.com/8876609033926980304/+/steps/ios_chrome_unittests__iPad_6th_Gen_iOS_12.4.1__on_iOS-12.4.1/0/logs/ChromeWebClientTest.WKWebViewEarlyPageScriptAccessibility/0

TBR: gambard
Bug: 1098758
Change-Id: I0b86000acb8b6394fa3b5649903c772f1d3756f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2264412Reviewed-by: Jérôme Lebel <jlebel@chromium.org>
Commit-Queue: Jérôme Lebel <jlebel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#781870}

ios/chrome/browser/web/chrome_web_client_unittest.mm

@@ -124,7 +124,8 @@ TEST_F(ChromeWebClientTest, UserAgent) {
 }
 
 // Tests that ChromeWebClient provides accessibility script for WKWebView.
-TEST_F(ChromeWebClientTest, WKWebViewEarlyPageScriptAccessibility) {
+// TODO(crbug.com/1098758): Test flaky.
+TEST_F(ChromeWebClientTest, FLAKY_WKWebViewEarlyPageScriptAccessibility) {
   // Chrome scripts rely on __gCrWeb object presence.
   WKWebView* web_view = web::BuildWKWebView(CGRectZero, browser_state());
   web::test::ExecuteJavaScript(web_view, @"__gCrWeb = {};");