Revert "Remove (unnecessary?) flush in RenderYUVAToRGBA"

This reverts commit 49b293a.

Reason for revert: speculative to fix chrome roll

Original change's description:
> Remove (unnecessary?) flush in RenderYUVAToRGBA
> 
> I don't think we need a flush here.
> 
> Change-Id: I87bf41b491835578e7df9bea4e15ea24dd551c1e
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/196645
> Reviewed-by: Greg Daniel <[email protected]>
> Commit-Queue: Robert Phillips <[email protected]>

[email protected],[email protected],[email protected]

Change-Id: I49305a5a2bcd5fe112b5f55b64511fd05d6c8548
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/197305
Reviewed-by: Mike Reed <[email protected]>
Commit-Queue: Mike Reed <[email protected]>
Auto-Submit: Mike Reed <[email protected]>

Author: reed-at-google

src/image/SkImage_GpuBase.cpp

@@ -361,6 +361,10 @@ bool SkImage_GpuBase::RenderYUVAToRGBA(GrContext* ctx, GrRenderTargetContext* re
     paint.addColorFragmentProcessor(std::move(fp));
 
     renderTargetContext->drawRect(GrNoClip(), std::move(paint), GrAA::kNo, SkMatrix::I(), rect);
+
+    // DDL TODO: in the promise image version we must not flush here
+    ctx->priv().flushSurfaceWrites(renderTargetContext->asSurfaceProxy());
+
     return true;
 }