Implement cross-thread MediaSource attachments #1663

Job	Run time
raspi-2-skia / initialize	33s
raspi-2 / initialize	51s
raspi-2 / docker-build-image	3m 27s
raspi-2 / docker-unittest-image	0s
raspi-2-skia / docker-build-image	3m 13s
raspi-2-skia / docker-unittest-image	0s
raspi-2-skia / raspi-2-skia_devel	11m 28s
raspi-2-skia / raspi-2-skia_debug	16m 56s
raspi-2-skia / raspi-2-skia_qa	10m 9s
raspi-2-skia / raspi-2-skia_gold	9m 46s
raspi-2 / raspi_devel	11m 49s
raspi-2 / raspi_debug	16m 35s
raspi-2 / raspi_qa	10m 1s
raspi-2 / raspi_gold	9m 54s
raspi-2 / sbversion-15_devel	11m 13s
raspi-2 / sbversion-15_debug	15m 46s
raspi-2 / sbversion-15_qa	10m 8s
raspi-2 / sbversion-15_gold	9m 31s
raspi-2 / sbversion-16_devel	11m 50s
raspi-2 / sbversion-16_debug	15m 34s
raspi-2 / sbversion-16_qa	10m 53s
raspi-2 / sbversion-16_gold	10m 22s
raspi-2-skia / ${{ matrix.name }}_${{ matrix.shard }}_test	0s
raspi-2-skia / ${{ matrix.name }}_on_device_${{ matrix.shard }}	0s
raspi-2 / ${{ matrix.name }}_${{ matrix.shard }}_test	0s
raspi-2 / ${{ matrix.name }}_on_device_${{ matrix.shard }}	0s
	3h 19m 59s

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