Remove ScopedObserver from base.

ScopedObserver is being deprecated in favor of two new classes:
- base::ScopedObservation for observers that only ever observe
  a single source.
- base::ScopedMultiSourceObservation for observers that do or may
  observe more than a single source.

Bug: 1145565
Change-Id: I3169858882de4efb16a63a847940a9e3bc249d96
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2944549
Reviewed-by: Daniel Cheng <[email protected]>
Commit-Queue: Sigurður Ásgeirsson <[email protected]>
Cr-Commit-Position: refs/heads/master@{#890208}
diff --git a/base/scoped_observation.h b/base/scoped_observation.h
index 909aafa..305c99c 100644
--- a/base/scoped_observation.h
+++ b/base/scoped_observation.h
@@ -15,8 +15,7 @@
 // where an observer observes a single source only.
 //
 // Use base::ScopedMultiSourceObservation for objects that observe multiple
-// sources. This class and base::ScopedMultiSourceObservation replace
-// ScopedObserver.
+// sources.
 //
 // When ScopedObservation is destroyed, it removes the registered observation,
 // if any. Basic example (as a member variable):