DevTools: fix http/tests/devtools/workers-on-navigation.js

The test was adding observer too late.

TBR=dgozman
BUG=700374

Change-Id: Ibbe66b9fa8234142aca0ddeef6dbd8c78fc358cb
Reviewed-on: https://chromium-review.googlesource.com/828142Reviewed-by: Andrey Lushnikov <lushnikov@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Andrey Lushnikov <lushnikov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524244}

third_party/WebKit/LayoutTests/TestExpectations

@@ -2984,7 +2984,6 @@ crbug.com/696407 [ Linux ] external/wpt/html/browsers/history/the-location-inter
 
 # Sheriff failures 2017-03-10
 crbug.com/741210 [ Mac ] inspector-protocol/emulation/device-emulation-restore.js [ Failure ]
-crbug.com/700374 [ Win ] http/tests/devtools/workers-on-navigation.js [ Failure Pass ]
 
 # Sheriff failures 2017-03-21
 crbug.com/703518 http/tests/devtools/tracing/worker-js-frames.js [ Failure Pass ]

third_party/WebKit/LayoutTests/http/tests/devtools/workers-on-navigation.js

@@ -31,8 +31,8 @@
       }
     }
   };
-  await TestRunner.navigatePromise('resources/workers-on-navigation-resource.html');
   SDK.targetManager.observeTargets(observer);
+  await TestRunner.navigatePromise('resources/workers-on-navigation-resource.html');
   await TestRunner.evaluateInPagePromise('startWorker()');
   await TestRunner.reloadPagePromise();
   navigated = true;