Add method to help us know whether audio block is audible or not, so that we won't
show the sound indicator for silent web audio.
Differential Revision: https://phabricator.services.mozilla.com/D7819
--HG--
extra : moz-landing-system : lando
This makes an AudioBlock valid for code testing mBufferFormat without IsNull(),
without the need for explicit SetNull().
This is useful so that setting AudioNodeStream::mLastChunks each iteration is
not required for inactive nodes.
--HG--
extra : rebase_source : 171b7339bfe91e15b070f03bd5e0b784b58a217a
Making the conversion constructor explicit means that it will be obvious if
a temporary is created to pass an AudioChunk as an AudioBlock parameter.
--HG--
extra : rebase_source : 54bf8acdb42499a0e0d66cfc138ff6fb6f1ef4da
In a subsequent patch, AllocateAudioBlock will become part of an AudioBlock
class derived from AudioChunk and used for AudioNodeStream members.
--HG--
extra : rebase_source : a3bfde8345995865c6f8e46abed24f008c112702
