weave: migrate away from base::ScopedObserver base:ScopedObserver will be removed at libchrome r890208. It should be replaced by either base::ScopedObservation or base::ScopedMultiSourceObservation. Note all usages in libweave belongs to the former since they only observe one source. BUG=b:198993705 TEST=FEATURES=test emerge-hatch libweave Change-Id: Ia35cd0a82dd936c4a0655e49b46b16579093ebf1 Reviewed-on: https://chromium-review.googlesource.com/c/weave/libweave/+/3224843 Tested-by: Grace Cham <[email protected]> Reviewed-by: Hidehiko Abe <[email protected]> Commit-Queue: Grace Cham <[email protected]>