DNR tests: Use ExtensionTestMessageListener to detect background page load.

This CL changes declarative net request browsertests to use
ExtensionTestMessageListener to detect background page load instead of relying
on the notification system, which is deprecated. Also, add a
extension_id_for_message() method to ExtensionTestMessageListener. This is
useful to detect the extension id, the message was received from.

BUG=696822

Change-Id: Ie1fc74d547d4ee417c5f6b2d5a7082adbc9d164a
Reviewed-on: https://chromium-review.googlesource.com/1034155
Commit-Queue: Karan Bhatia <karandeepb@chromium.org>
Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555005}

• chrome/browser/extensions/api/declarative_net_request/declarative_net_request_browsertest.cc[diff]
• extensions/common/api/declarative_net_request/test_utils.cc[diff]
• extensions/common/api/declarative_net_request/test_utils.h[diff]
• extensions/test/extension_test_message_listener.cc[diff]
• extensions/test/extension_test_message_listener.h[diff]