Revert "Disable flaky AssistantOptInFlowTest.WAADisabledByPolicy"

This reverts commit 2de3b7e6.

Reason for revert: root cause should be fixed by CL:1868769

Original change's description:
> Disable flaky AssistantOptInFlowTest.WAADisabledByPolicy
> 
> TBR=updowndota
> 
> Bug: 1013824
> Change-Id: Ibbeb425fe2511f0dadaa93bb4dd3cbfde476877b
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1857130
> Reviewed-by: David Roger <droger@chromium.org>
> Commit-Queue: David Roger <droger@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#705491}

TBR=droger@chromium.org,updowndota@chromium.org

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

Bug: 1013824
Change-Id: I0cd2c2b8119876ff2ed7b1866236783611b65a2d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1892593Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Commit-Queue: Roman Sorokin [CET] <rsorokin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711210}

chrome/browser/chromeos/login/screens/assistant_optin_flow_screen_browsertest.cc

@@ -1001,8 +1001,7 @@ IN_PROC_BROWSER_TEST_F(AssistantOptInFlowTest,
   EXPECT_TRUE(prefs->GetBoolean(assistant::prefs::kAssistantContextEnabled));
 }
 
-// This test is flaky. See https://crbug.com/1013824
-IN_PROC_BROWSER_TEST_F(AssistantOptInFlowTest, DISABLED_WAADisabledByPolicy) {
+IN_PROC_BROWSER_TEST_F(AssistantOptInFlowTest, WAADisabledByPolicy) {
   assistant_settings_->set_consent_ui_flags(
       FakeAssistantSettings::CONSENT_UI_FLAG_WAA_DISABLED_BY_POLICY);