Enable PolicyTest.FileURLBlacklist for Mac.

The test was disabled with https://codereview.chromium.org/608153002 as
flaky on Mac. It does not seem to be a case anymore.

BUG=339240
TEST=unit_tests --gtest_filter="PolicyTest.FileURLBlacklist"

Change-Id: I4f67e001ee73b75152895960e37af5e6c98ce65e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2550028Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Commit-Queue: Artem Sumaneev <asumaneev@google.com>
Cr-Commit-Position: refs/heads/master@{#829250}

chrome/browser/policy/url_blacklist_policy_browsertest.cc

@@ -308,13 +308,7 @@ IN_PROC_BROWSER_TEST_F(PolicyTest, URLBlacklistServerRedirect) {
             browser()->tab_strip_model()->GetActiveWebContents()->GetTitle());
 }
 
-#if defined(OS_MAC)
-// http://crbug.com/339240
-#define MAYBE_FileURLBlacklist DISABLED_FileURLBlacklist
-#else
-#define MAYBE_FileURLBlacklist FileURLBlacklist
-#endif
-IN_PROC_BROWSER_TEST_F(PolicyTest, MAYBE_FileURLBlacklist) {
+IN_PROC_BROWSER_TEST_F(PolicyTest, FileURLBlacklist) {
   // Check that FileURLs can be blacklisted and DisabledSchemes works together
   // with URLblacklisting and URLwhitelisting.