Fix teardown use-after-free in MediaEngagementServiceTest

Destroy the MediaEngagementService object later. Should fix the
ToTLinuxUBSanVptr bot on the clang ToT waterfall.

R=beccahughes@chromium.org,mlamouri@chromium.org
BUG=chromium:827609

Change-Id: I9570279416a39d1018f714762d3df871cf2ce07a
Reviewed-on: https://chromium-review.googlesource.com/1015770Reviewed-by: Becca Hughes <beccahughes@chromium.org>
Commit-Queue: Reid Kleckner <rnk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551497}

chrome/browser/media/media_engagement_service_unittest.cc

@@ -151,8 +151,8 @@ class MediaEngagementServiceTest : public ChromeRenderViewHostTestHarness {
 
   void TearDown() override {
     service_->Shutdown();
-    service_.reset();
     ChromeRenderViewHostTestHarness::TearDown();
+    service_.reset();
   }
 
   void AdvanceClock() {