[graphite] Add build flag to enable trace events for pipeline + RP labels

* Helpful when debugging to be able to (dis- / en-)able this flag without having to add a #define somewhere or otherwise introduce a temporary unnecessary code change.

Change-Id: I3f50d38fb2cd8f3c721fb9e8f89d47335f5f00f6
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1017297
Commit-Queue: Nicolette Prevost <nicolettep@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
2 files changed