Remove navigation_service_unittests, along with the service itself.

services/navigation/README.md

-Navigation Service
-====
-
-### Overview
-
-Navigation is a Chrome Foundation Service that provides the concept of a
-"navigable view". A navigable view is an application host that can be
-navigated to different states in sequence, like a traditional "webview" on other
-platforms.
-
-The Navigation service maintains a tree of frames, within which different
-applications run. Each navigation state corresponds to a potentially different
-tree structure with different applications run in each frame.
-
-In theory, the Navigation service should not care about the specifics of the
-application hosted in each frame, though in practice today because the
-implementation is provided by //content, all frames are instances of Blink.
-
-When an application hosted by a view requires a capability not intrinsic to the
-container, it requests via the ViewClient that the embedder provide it. For some
-capabilities the grant action is simply providing some metadata directly. For
-others, they are modeled by the acquisition of a Mojo message pipe handle
-(interface) to some service that exposes the capability. In this latter case it
-is possible, again in theory, that the middleware layer (Navigation service) not
-know about the specifics of the capability and just forward the request from the
-frame renderer to the Navigation service client where it will either be bound or
-the message pipe closed.
-
-A client of the Navigation service may embed a visual representation of a view into
-its own rendering using Mus. The View interface provides a means to acquire a
-mus.mojom.WindowTreeClient that can be embedded in a mus::Window created by the
-client. The Navigation service may create its own mus::Windows within this window,
-but these will not be visible to the client according to Mus security policy.
-
-### Details
-
-Concretely, this service is a //content client, using the Content Public API
-to construct a WebContents for every request for a navigation::mojom::View.
-The class that binds these two together is //services/navigation/view_impl.h.
-This class implements the WebContentsDelegate (& other) interfaces to learn
-about state changes within the Content layer & transmit them to the Navigation
-Service client via navigation::mojom::ViewClient.
-
-Note that the //content client provided here
-(in //services/navigation/content_client) is very simple, even simpler than
-Content Shell's. This is not intended to be a long term solution, just a
-mechanism to avoid changing Content. If we were to deploy this code in //chrome,
-we might want to expose these interfaces from content directly.
-
-Lastly, the desired end state of the service-refactoring is that //content
-ceases to exist, so at that point it's conceivable that //content types that
-are used to implement these interfaces end up in this directory. But that's a
-long way off.
-
-### Usage
-
-For convenience, as synchronizing some navigable view state is tricky, a client
-library is provided in //services/navigation/public/cpp. The types exposed in
-this library mimic conventional platform WebView types.
-
-    std::unique_ptr<navigation::View> view(new navigation::View);
-    ViewDelegateImpl delegate;
-    ViewObserverImpl observer;
-    view->set_delegate(&delegate);
-    view->AddObserver(&observer);
-    view->EmbedInWindow(some_mus_window);
-    view->NavigateToURL(GURL("http://www.chromium.org/"));