relates to #8841

Issue Context

• Issue is visible on the amp-ima-video AMP By Example.

• Source code for the <amp-ima-video> can be found in ads/google/imaVideo.js.

• Currently, we are designing/considering a rebuild of video within AMP to use Video.js, so smaller issues will be considered as we don't want to dedicate too much time on the older component version.

Issue Description

Currently there is no functioning Mute/Unmute button on the amp-ima-video extension.