Install URLLoaderInterceptor on ErrorPageOfflineTest

ErrorPageOfflineTestWithAllowDinosaurFalse.CheckEasterEggIsDisabled requires
http://mock.request.failed to be handled. This CL installs a default interceptor
which handles this request.

Bug: 776589
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_mojo
Change-Id: Ic5b2dca7b3d46b20368094c677ea85456a39b016
Reviewed-on: https://chromium-review.googlesource.com/847917
Commit-Queue: Doug Turner <dougt@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526868}

chrome/browser/net/errorpage_browsertest.cc

@@ -1520,8 +1520,15 @@ class ErrorPageOfflineTest : public ErrorPageTest {
     BrowserThread::PostTask(
         BrowserThread::IO, FROM_HERE,
         base::BindOnce(&ErrorPageOfflineTest::InstallMockInterceptors));
+    url_loader_interceptor_ =
+        std::make_unique<content::URLLoaderInterceptor>(base::BindRepeating(
+            [](content::URLLoaderInterceptor::RequestParams* params) {
+              return false;
+            }));
   }
 
+  void TearDownOnMainThread() override { url_loader_interceptor_.reset(); }
+
  protected:
   void SetUpInProcessBrowserTestFixture() override {
 #if defined(OS_CHROMEOS)
@@ -1604,6 +1611,7 @@ class ErrorPageOfflineTest : public ErrorPageTest {
 
   // Mock policy provider for both user and device policies.
   policy::MockConfigurationPolicyProvider policy_provider_;
+  std::unique_ptr<content::URLLoaderInterceptor> url_loader_interceptor_;
 };
 
 class ErrorPageOfflineTestWithAllowDinosaurTrue : public ErrorPageOfflineTest {

testing/buildbot/filters/mojo.fyi.network_browser_tests.filter

@@ -611,7 +611,6 @@
 
 # Switch test from using a custom net::URLRequestFileJob to using a test
 # URLLoaderFactory via SetNetworkFactoryForTesting.
--ErrorPageOfflineTestWithAllowDinosaurFalse.CheckEasterEggIsDisabled
 -ErrorPageSniffTest.SniffSmallHttpErrorResponseAsDownload
 -DNSErrorPageTest.StaleCacheStatus