Prompt for granting permission to access USB devices.

extensions::DevicePermissionsPrompt may be implemented by an embedder
of the extensions system to provide a way to prompt the user for
permission to access a set of USB devices. This is expandable to other
device types. Implementations for Chrome using the Views framework
(for Chrome OS, Linux and Windows) and Cocoa (for OS X) are provided
in this change.

Screenshots are attached to bug 420317.


Review URL:

Cr-Commit-Position: refs/heads/master@{#299983}
19 files changed