experimental/editor: clean up BUILD.gn

Place build rules in experimental/editor/BUILD.gn to keep the top-level
BUILD.gn file smaller.

Change-Id: Idf22aec6f86bc510f4528fe0296bc1bea69f3df2
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/230740
Commit-Queue: Hal Canary <halcanary@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
Auto-Submit: Hal Canary <halcanary@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
2 files changed