Add audio controls for individual tabs

[taravancil]

It would be great to be able to indicate which tabs are playing audio, and to be able to mute the audio from the tab:

Here are the isolated tasks we'll need to complete in order to make this happen:

• Detect when audio is being played, and identify which tab is playing it
• Implement muting/unmuting audio in a given tab
• Wire up the tab UI

If anyone is interested in helping, Brave's codebase might be helpful. They implemented this feature and they also use Electron.

[pfrazee]

We need to update electron to support telling us when audio is playing.