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In general, image format conversion is time-consuming task.
ImageProcessor, the interface for image conversion, should offload the
conversion tasks to a different thread to avoid occupying the working
thread for long time. Originally, thread hopping is handled by each
implementation. It makes implementations complicated and error-prone.

In this CL, we refactor the structure of ImageProcessor. We add a new
interface "ImageProcessorBackend". Each implementation change to
inherit this interface. The interface only works on backend sequence.
ImageProcessor becomes a concrete class instead of a interface. It
has a ImageProcessorBackend instance, and is in charge of handling
sequence hopping between client sequence and backend sequence.
ImageProcessor also guarantees to call or destroy the client's
callback on the client sequence.

Bug: 1028889
Test: run vda_tests on kukui and hana
Test: build media/gpu/{chromeos,vaapi,v4l2}

Change-Id: I0f7774bc62adc7d4b4adf68ab33d95d9013656be
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1942600
Commit-Queue: Chih-Yu Huang <akahuang@chromium.org>
Reviewed-by: Alexandre Courbot <acourbot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#725476}