Remove unnecessary flush in RenderYUVAToRGBA (take 2)

There is no real reason to flush here. If clients need to access the results immediately they should be flushing at the API level.

A reland of: https://skia-review.googlesource.com/c/skia/+/196645

It needs to land after the Chrome CL:

https://chromium-review.googlesource.com/c/chromium/src/+/1560554 (Flush pending work after calls to YUVGrBackendTexturesToSkImage)

Change-Id: I230d7433b5c1b72d8f50fd475033c942826eda94
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/206688
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
1 file changed