[Maintenance] Consolidated fDrawablePicts with DrawDrawable

Now that fDrawablePicts exists within SkPicture, we can update NestedApproxOpCounter to use fDrawablePicts for its op counting.

Before, SkRecords::DrawDrawable would be counted as 1 operation (op) (i.e. operator()(const T& op) { fCount += 1 }. There was no override for SkRecords::DrawDrawable, and  no logic for parsing the op count for each drawable.

We can pass in fDrawablePicts into NestedApproxOpCounter to call approximateOpCount on each drawablePict according to its index.

This is in response to this suggestion on an earlier CL: https://skia-review.git.corp.google.com/c/skia/+/1337836/comment/8d6fbbce_c92cd00d/

Change-Id: I56c1f34341ec25c70e44f37231f49089af35d9d4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1344256
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Alexis Cruz-Ayala <alexisdavidc@google.com>