Add an empty Android implementation of the PhysicalWebDataSource

The Physical Web data source collects information about nearby devices
broadcasting URLs and makes the list of URLs and associated metadata
available to components within Chrome. This change adds an empty
implementation of the data source in Chrome for Android and makes it
available from a BrowserProcess.

BUG=636490

Review-Url: https://codereview.chromium.org/2361603002
Cr-Commit-Position: refs/heads/master@{#423754}
8 files changed