Elevate the target list of Ganesh tasks to base class

A follow up CL will use the availability of this information - the full
list of targets of a GrRenderTask – to enable faster storage of the
lastRenderTask association for a given surface proxy in a given drawing
manager.

Bug: skia:10320
Change-Id: I3eb3276b483a7f09481774896a024172b73a4c84
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/296729
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Adlai Holler <adlai@google.com>
diff --git a/src/gpu/GrTransferFromRenderTask.h b/src/gpu/GrTransferFromRenderTask.h
index 2126ae8..93a0f7a 100644
--- a/src/gpu/GrTransferFromRenderTask.h
+++ b/src/gpu/GrTransferFromRenderTask.h
@@ -28,7 +28,7 @@
 
 private:
     bool onIsUsed(GrSurfaceProxy* proxy) const override {
-        SkASSERT(!fTargetView.proxy());
+        SkASSERT(0 == this->numTargets());
         return proxy == fSrcProxy.get();
     }
     // If fSrcProxy is uninstantiated at flush time we simply will skip doing the transfer.