Fix WebContents leak in MetricsWebContentsObserverTest.

Leaked WebContents prevent the destruction of aura::Env with
the upcoming aura::WindowOcclusionTracker.

Bug: 
Change-Id: I259095931f72dcb82b567c38d3620c7ee981a0c8
Reviewed-on: https://chromium-review.googlesource.com/673547
Commit-Queue: Francois Doray <fdoray@chromium.org>
Reviewed-by: Bryan McQuade <bmcquade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#502949}

chrome/browser/page_load_metrics/metrics_web_contents_observer_unittest.cc

@@ -1012,9 +1012,9 @@ TEST_F(MetricsWebContentsObserverTest,
   // committed RFH are ignored. There isn't a way to hold on to an old RFH (it
   // gets cleaned up soon after being navigated away from) so instead we use an
   // RFH from another WebContents, as a way to simulate the desired behavior.
-  content::WebContents* other_web_contents =
+  std::unique_ptr<content::WebContents> other_web_contents(
       content::WebContentsTester::CreateTestWebContents(browser_context(),
-                                                        nullptr);
+                                                        nullptr));
   observer()->OnRequestComplete(
       GURL("http://www.other.com/"), net::HostPortPair(),
       other_web_contents->GetMainFrame()->GetFrameTreeNodeId(),