Implement cross-thread MediaSource attachments #1497

Job	Run time
raspi-2 / initialize	44s
raspi-2-skia / initialize	43s
raspi-2-skia / docker-unittest-image	0s
raspi-2-skia / docker-build-image	2m 27s
raspi-2 / docker-build-image	2m 26s
raspi-2 / docker-unittest-image	0s
raspi-2-skia / raspi-2-skia_devel	11m 24s
raspi-2-skia / raspi-2-skia_debug	16m 11s
raspi-2-skia / raspi-2-skia_qa	9m 24s
raspi-2-skia / raspi-2-skia_gold	9m 27s
raspi-2 / raspi_devel	11m 18s
raspi-2 / raspi_debug	15m 19s
raspi-2 / raspi_qa	9m 31s
raspi-2 / raspi_gold	9m 42s
raspi-2 / sbversion-15_devel	11m 40s
raspi-2 / sbversion-15_debug	15m 8s
raspi-2 / sbversion-15_qa	9m 30s
raspi-2 / sbversion-15_gold	9m 34s
raspi-2 / sbversion-16_devel	12m 44s
raspi-2 / sbversion-16_debug	15m 31s
raspi-2 / sbversion-16_qa	9m 46s
raspi-2 / sbversion-16_gold	9m 13s
raspi-2 / ${{ matrix.name }}_${{ matrix.shard }}_test	0s
raspi-2 / ${{ matrix.name }}_on_device_${{ matrix.shard }}	0s
raspi-2-skia / ${{ matrix.name }}_${{ matrix.shard }}_test	0s
raspi-2-skia / ${{ matrix.name }}_on_device_${{ matrix.shard }}	0s
	3h 11m 42s

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