Add gn plumbing for mdbviz tool

Change-Id: I06e6b63c2742da069f48ff5d7defafc63a485af7
Reviewed-on: https://skia-review.googlesource.com/41842
Reviewed-by: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
7 files changed