[debugger] Add /healthz handler.
[debugger] Add /healthz handler.

Bug: skia:
Change-Id: Iccaa081edbaf846f066325ddf1cf88ea8f5236c0
Reviewed-on: https://skia-review.googlesource.com/135144
Reviewed-by: Ben Wagner <3ef7217be91069877d94f7907ce5479000772cd3@google.com>
Commit-Queue: Joe Gregorio <b9cf2471ff6d33504cafe8e6f356b732b2cadced@google.com>

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot