Revert "Disabling flaky test PolicyPrefsTest.PolicyToPrefsMapping on Windows."

This reverts commit 788a292e.

Reason for revert: Re-enabling the PolicyPrefsTest.PolicyToPrefsMapping test, as the flaky DCHECK has been disabled in https://crbug.com/826744.

Original change's description:
> Disabling flaky test PolicyPrefsTest.PolicyToPrefsMapping on Windows.
> 
> TBR=emaxx@chromium.org
> TBR=pastarmovj@chromium.org
> 
> Bug: 822975
> Change-Id: I03062d8eb1c8c3221cf07eb08587e4d9c97ed5e2
> Reviewed-on: https://chromium-review.googlesource.com/968507
> Reviewed-by: Vadym Doroshenko <dvadym@chromium.org>
> Commit-Queue: Vadym Doroshenko <dvadym@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#544023}

TBR=pastarmovj@chromium.org,emaxx@chromium.org,dvadym@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 822975
Change-Id: I5721a14572d7f7a5b019fd5508ced218f19790a5
Reviewed-on: https://chromium-review.googlesource.com/986097Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Commit-Queue: Maksim Ivanov <emaxx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546994}

chrome/browser/policy/policy_prefs_browsertest.cc

@@ -455,15 +455,9 @@ class PolicyPrefsTest : public InProcessBrowserTest {
   MockConfigurationPolicyProvider provider_;
 };
 
-// Flaky on Win only. https://crbug.com/822975
-#if defined(OS_WIN)
-#define MAYBE_PolicyToPrefsMapping DISABLED_PolicyToPrefsMapping
-#else
-#define MAYBE_PolicyToPrefsMapping PolicyToPrefsMapping
-#endif
 // Verifies that policies make their corresponding preferences become managed,
 // and that the user can't override that setting.
-IN_PROC_BROWSER_TEST_F(PolicyPrefsTest, MAYBE_PolicyToPrefsMapping) {
+IN_PROC_BROWSER_TEST_F(PolicyPrefsTest, PolicyToPrefsMapping) {
   Schema chrome_schema = Schema::Wrap(GetChromeSchemaData());
   ASSERT_TRUE(chrome_schema.valid());
   PrefService* local_state = g_browser_process->local_state();