[headless] Ensure devtooled tests call Disable/RemoveObserver.

headless/public/util/dom_tree_extractor_browsertest.cc

Index: headless/public/util/dom_tree_extractor_browsertest.cc
diff --git a/headless/public/util/dom_tree_extractor_browsertest.cc b/headless/public/util/dom_tree_extractor_browsertest.cc
index 808a10b614e20dcc3618f04b2f0fc4c5796d851a..22b237960c531047398bf1d7291862cc040e431c 100644
--- a/headless/public/util/dom_tree_extractor_browsertest.cc
+++ b/headless/public/util/dom_tree_extractor_browsertest.cc
@@ -49,6 +49,7 @@ class DomTreeExtractorBrowserTest : public HeadlessAsyncDevTooledBrowserTest,
   }
 
   void OnLoadEventFired(const page::LoadEventFiredParams& params) override {
+    devtools_client_->GetPage()->Disable();
     devtools_client_->GetPage()->RemoveObserver(this);
 
     extractor_.reset(new DomTreeExtractor(devtools_client_.get()));