Improve flush-time op spew (esp. for DDL tasks)

This CL also centralizes how the SkSurfaceProxy is output.

Change-Id: Ibdba1535e65ef21ce206778a8d757ee341334ec0
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/352081
Reviewed-by: Adlai Holler <adlai@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
diff --git a/src/gpu/GrDrawingManager.cpp b/src/gpu/GrDrawingManager.cpp
index 8ca4553..d6eee6c 100644
--- a/src/gpu/GrDrawingManager.cpp
+++ b/src/gpu/GrDrawingManager.cpp
@@ -303,7 +303,9 @@
                             startIndex, stopIndex, 0, fDAG.count());
     for (int i = startIndex; i < stopIndex; ++i) {
         if (fDAG[i]) {
-            fDAG[i]->dump(true);
+            SkString label;
+            label.printf("task %d/%d", i, fDAG.count());
+            fDAG[i]->dump(label, {}, true, true);
         }
     }
 #endif