Register OOP Mojo apps with the browser

This changes the embedded Mojo shell to only load out-of-process
Mojo apps that have been registered by the embedder. The purpose
of this is two-fold:

 1. It prevents unknown app requests from spinning up a useless
    utility process that will just die immediately.
 2. It provides a way to register per-app process names to be used
    when loading apps out-of-process.

BUG=492422,526808,526810

Review URL: https://codereview.chromium.org/1311673013

Cr-Commit-Position: refs/heads/master@{#346710}

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