[sensors] Add GetDefaultConfiguration() to generic sensor mojo interface.

This patch adds 'GetDefaultConfiguration' method to the generic sensor interface.
It is required in cases when client (e.g. blink bindings) doesn't know what
configuration to use for sensor initialization or when configuration that is
provided to the client is incomplete.

BUG=606766

Review-Url: https://codereview.chromium.org/2288103002
Cr-Commit-Position: refs/heads/master@{#415261}
4 files changed