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This is an initial Blimp integration test that exercises navigation history. The purpose is to test the integration with WebContentsImpl and NavigationControllerImpl, as well as the proper flow of Blimp's NavigationStateChangeMessage protobuf across the client-engine network connection.

The test visits two different web pages and then goes back to the first one.

Additional supporting changes:
* Moving some setup code from the initial LoadUrl test into SetUpOnMainThread.
* Helper method ExpectPageLoad, which sets mock expectations for a page load, and can be used with the helper method RunAndVerify to wait for Blimp to load a page.
* Always storing the latest page title when it changes on the client side, making it easier to assert about what page was most recently loaded.

Review-Url: https://codereview.chromium.org/2272513003
Cr-Commit-Position: refs/heads/master@{#414755}