DRM: Support for robustness attribute

[dsilhavy]

Description

The Content Protection IOP guidelines available here define support for the @robustness attribute:

The ContentProtection descriptor may include a @robustness attribute. The value of this attribute is DRM
specific. It announces what robustness level is expected from the DRM system for the representations that are part of
the adaptation set. It allows the DASH client to make a more educated choice when selecting a DRM system for playing
back content or to not consider some adaptation sets when there is no suitable DRM system on the device.
In order to leverage this value, the DASH client needs to know the robustness level of the different DRM systems
supported by the platform. There is no recommended API for getting this information.

NOTE: If using W3C EME [8], the @robustness value can be compared to the value of the robustness member
of MediaKeySystemMediaCapability for selecting a key system to activate. The DASH client selects then
a key system with the requested robustness level.