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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}