Ignores the default favicon request in ChromeServiceWorkerFetchTest

Currently if the default favicon request is caught by the testing service worker
the test fails.
This is causing the test flakiness.
To fix this flakiness, this CL change the fetch event handler of the service
worker to ignore the default favicon request.

Bug: 912543
Change-Id: Icdddd902a4547361320f43db5b9090270ba74509
Reviewed-on: https://chromium-review.googlesource.com/c/1370160
Commit-Queue: Tsuyoshi Horo <horo@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615470}

chrome/browser/chrome_service_worker_browsertest.cc

@@ -282,6 +282,15 @@ class ChromeServiceWorkerFetchTest : public ChromeServiceWorkerTest {
               "  event.waitUntil(self.clients.claim());"
               "};"
               "this.onfetch = function(event) {"
+              // Ignore the default favicon request. The default favicon request
+              // is sent after the page loading is finished, and we can't
+              // control the timing of the request. If the request is sent after
+              // clients.claim() is called, fetch event for the default favicon
+              // request is triggered and the tests become flaky. See
+              // https://crbug.com/912543.
+              "  if (event.request.url.endsWith('/favicon.ico')) {"
+              "    return;"
+              "  }"
               "  event.respondWith("
               "      self.clients.matchAll().then(function(clients) {"
               "          clients.forEach(function(client) {"