Fix flaky CertificateProviderApiTest.Basic test.

The flakiness was caused by waiting for a wrong number of JavaScript
test results, which resulted in inconsistency between events that were
expected to happen and the events that have actually happened. In a
detailed manner, the reportCertificates callback in file basic.js is
wrapped through chrome.test.callbackPass method, which itself calls
chrome.test.succeed method; meanwhile the C++ code in
certificate_provider_apitest.cc didn't expect this chrome.test.succeed
call, and it behaved as if the
chrome.certificateProvider.onSignDigestRequested event handler was
already executed - which is not always true and causes flakiness.

BUG=576364

Review URL: https://codereview.chromium.org/1632053002

Cr-Commit-Position: refs/heads/master@{#371501}

chrome/browser/extensions/api/certificate_provider/certificate_provider_apitest.cc

@@ -159,13 +159,7 @@ class CertificateProviderApiTest : public ExtensionApiTest {
 
 }  // namespace
 
-// See crbug.com/576364 for details
-#if defined(OS_CHROMEOS)
-#define MAYBE_Basic DISABLED_Basic
-#else
-#define MAYBE_Basic Basic
-#endif
-IN_PROC_BROWSER_TEST_F(CertificateProviderApiTest, MAYBE_Basic) {
+IN_PROC_BROWSER_TEST_F(CertificateProviderApiTest, Basic) {
   // Start an HTTPS test server that requests a client certificate.
   net::SpawnedTestServer::SSLOptions ssl_options;
   ssl_options.request_client_certificate = true;
@@ -197,6 +191,9 @@ IN_PROC_BROWSER_TEST_F(CertificateProviderApiTest, MAYBE_Basic) {
   content::WebContents* const https_contents =
       browser()->tab_strip_model()->GetActiveWebContents();
 
+  VLOG(1) << "Wait for the extension to respond to the certificates request.";
+  ASSERT_TRUE(catcher.GetNextResult()) << catcher.message();
+
   VLOG(1) << "Wait for the extension to receive the sign request.";
   ASSERT_TRUE(catcher.GetNextResult()) << catcher.message();