media/gpu/test: ImageProcessorClient: Use bytes for width and height in libyuv::CopyPlane()

|width| is in bytes in libyuv::CopyPlane(). We formerly pass width in pixels.
This should matter when a pixel format is used whose pixel is composed of
more than one bytes.

Bug: None
Test: image_processor_test
Change-Id: I98e90be70c8d0128319172d4d19f3a8017b65d78
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1553129
Commit-Queue: Hirokazu Honda <hiroh@chromium.org>
Reviewed-by: Alexandre Courbot <acourbot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#648117}