maintain RenderTask dependencies through merge

If they are not maintained, then the merge leaves links to
deleted RenderTarget nodes. This creates confusion while
trying to debug.

Change-Id: I9d98a4ed2887573ddc5dfa2bae1bb22d30879c5a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/402641
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Adlai Holler <adlai@google.com>
3 files changed