Re-enable PolicyTest.ExtensionInstallBlacklistSelective.

This test was flaky on Windows but it was due to other problems at
shutdown. Re-enabling to determine if the problem still exists.

BUG=388340

Review URL: https://codereview.chromium.org/488953002

Cr-Commit-Position: refs/heads/master@{#290806}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@290806 0039d316-1c4b-4281-b951-d872f2087c98

chrome/browser/policy/policy_browsertest.cc

@@ -1524,9 +1524,7 @@ IN_PROC_BROWSER_TEST_F(PolicyTest, DownloadDirectory) {
   EXPECT_FALSE(base::PathExists(initial_dir.path().Append(file)));
 }
 
-// Flaky: http://crbug.com/388340
-IN_PROC_BROWSER_TEST_F(PolicyTest,
-                       DISABLED_ExtensionInstallBlacklistSelective) {
+IN_PROC_BROWSER_TEST_F(PolicyTest, ExtensionInstallBlacklistSelective) {
   // Verifies that blacklisted extensions can't be installed.
   ExtensionService* service = extension_service();
   ASSERT_FALSE(service->GetExtensionById(kGoodCrxId, true));