Set willReadFragmentPosition on XP when doing a DstCopy.

BUG=472872

Review URL: https://codereview.chromium.org/1068463004
diff --git a/src/gpu/GrXferProcessor.cpp b/src/gpu/GrXferProcessor.cpp
index 358a249..476394f 100644
--- a/src/gpu/GrXferProcessor.cpp
+++ b/src/gpu/GrXferProcessor.cpp
@@ -18,6 +18,7 @@
         fDstCopy.reset(dstCopy->texture());
         fDstCopyTextureOffset = dstCopy->offset();
         this->addTextureAccess(&fDstCopy);
+        this->setWillReadFragmentPosition();
     }
 }