Don't enable page domain to navigate in test

This resolves an inconsistency in a test, which may cause the test to
fail if a new event in the Page domain is added. The reasons is that
only if  devtools_client_->GetPage()->AddObserver() is called,
we call Domain::RegisterEventHandlersIfNeeded. This method adds the
Page's events to the  HeadlessDevToolsClientImpl::event_handlers_ map.

The test doesn't call devtools_client_->GetPage()->AddObserver() and
hence Page events are not registered.

Any events received during the test on the Page domain triggers
the NOTREACHED in
https://source.chromium.org/chromium/chromium/src/+/master:headless/lib/browser/headless_devtools_client_impl.cc;l=284;drc=8fb8160447dcb4e007b67be730b745a99570e1cb;bpv=1;bpt=1

Change-Id: Ie5fbf1e0cbf4658060c22ac6eee9d5fdacd38968
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2585086
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836994}

headless/test/headless_devtools_client_browsertest.cc

@@ -494,7 +494,6 @@ class HeadlessCrashObserverTest : public HeadlessAsyncDevTooledBrowserTest,
     devtools_client_->GetInspector()->GetExperimental()->AddObserver(this);
     devtools_client_->GetInspector()->GetExperimental()->Enable(
         inspector::EnableParams::Builder().Build());
-    devtools_client_->GetPage()->Enable();
     devtools_client_->GetPage()->Navigate(content::kChromeUICrashURL);
   }