The purpose of this bug is to move out the codes for seamless audio from DecodingState::Step().

If we're in seamless looping, ReaderProxy will not tell MDSM about EOS error which reader returned, so MDSM will stay at `decoding state` all the time. We can say that the ReaderProxy hides EOS in this case. When canceling seamless looping, if the playback position does not reach the EOS position yet and we had already hidden EOS, we need to create another EOS to notify MDSM the demuxing has ended. playback hidden fake position EOS EOS ----|---------|---------|-------> (Increasing timeline)

When we cancel seamless looping, we should discard the audio data whose time are later than the last audio frame's time in the audio track.

No need to check whether we need to dispatch looping event everytime when entering the DecodingState::Step().

We only have a interest in the data which is popped out from the front side of the queue.

This state is used to handle decoding when the media is in seamless looping. It will be responsible for (1) time adjustment (2) handle EOS and seek to the first sample

When we cancel seamless looping, we should discard audio data whose time are later than the last audio frame's time in the audio track.

We only have a interest in the data which is popped out from the front side of the queue.

We don't have any implementation for video seamless looping yet, so we only use 'loopingDecodingState' for audio for now.

Ensure we disconnect all Then() before exit the state.

Pushed by alwu@mozilla.com: https://hg.mozilla.org/integration/autoland/rev/8937fe137441 part1 : implement MDSM's 'LoopingDecodingState'. r=jya https://hg.mozilla.org/integration/autoland/rev/f47b8fef7cc7 part2 : implement state transition. r=jya https://hg.mozilla.org/integration/autoland/rev/c67e483b4ab4 part3 : remove time-adjustment related codes in ReaderProxy. r=jya https://hg.mozilla.org/integration/autoland/rev/2e61ba5a50f2 part4 : discard redudant looping data when cancel looping. r=jya https://hg.mozilla.org/integration/autoland/rev/1cd5c63b4182 part5 : rename PopEvent() to PopFrontEvent(). r=jya https://hg.mozilla.org/integration/autoland/rev/93cfc552f067 part6 : only enable 'loopingDecodingState' for audio seamless looping r=jya

https://hg.mozilla.org/mozilla-central/rev/8937fe137441 https://hg.mozilla.org/mozilla-central/rev/f47b8fef7cc7 https://hg.mozilla.org/mozilla-central/rev/c67e483b4ab4 https://hg.mozilla.org/mozilla-central/rev/2e61ba5a50f2 https://hg.mozilla.org/mozilla-central/rev/1cd5c63b4182 https://hg.mozilla.org/mozilla-central/rev/93cfc552f067