[MemSheriff] Add ASSERTs to ferret out test flakes.

SSLUITest::TestBrokenHTTPSReporting occasionally crashes around the
GetInterstitialPage() call on line 470. See e.g. http://build.chromium.org/p/chromium.memory.fyi/builders/Windows%20Browser%20%28DrMemory%20full%29%20%287%29/builds/1934.

Added ASSERTs to narrow down what is happening.

R=felt@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1495683002

Cr-Commit-Position: refs/heads/master@{#363596}

chrome/browser/ssl/ssl_browser_tests.cc

@@ -459,15 +459,18 @@ class SSLUITest
     ui_test_utils::NavigateToURL(browser, https_server_expired_.GetURL("/"));
 
     WebContents* tab = browser->tab_strip_model()->GetActiveWebContents();
+    ASSERT_TRUE(tab != nullptr);
     CheckAuthenticationBrokenState(tab, net::CERT_STATUS_DATE_INVALID,
                                    AuthState::SHOWING_INTERSTITIAL);
 
     scoped_ptr<SSLCertReporter> ssl_cert_reporter =
         certificate_reporting_test_utils::SetUpMockSSLCertReporter(
             &run_loop, expect_report);
 
+    ASSERT_TRUE(tab->GetInterstitialPage() != nullptr);
     SSLBlockingPage* interstitial_page = static_cast<SSLBlockingPage*>(
         tab->GetInterstitialPage()->GetDelegateForTesting());
+    ASSERT_TRUE(interstitial_page != nullptr);
     interstitial_page->SetSSLCertReporterForTesting(ssl_cert_reporter.Pass());
 
     EXPECT_EQ(std::string(), GetLatestHostnameReported());