Deprecate ScopedObserver.

It's replaced by base::ScopedObservation for observing a single source,
or base::ScopedMultiSourceObservation for observing multiple sources.

Bug: 1145565
Change-Id: Iadc2e3efee7860e1b17b86b077d4c6a2566b6a95
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2527541
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825580}

PRESUBMIT.py

@@ -1081,6 +1081,16 @@ _BANNED_CPP_FUNCTIONS = (
         r'^base/tracing/.*',
       ),
     ),
+    (
+      r'/\bScopedObserver',
+      (
+          'ScopedObserver is deprecated.',
+          'Please use base::ScopedObservation for observing a single source,',
+          'or base::ScopedMultiSourceObservation for observing multple sources',
+      ),
+      False,
+      (),
+    ),
 )
 
 # Format: Sequence of tuples containing:

base/scoped_observer.h

@@ -13,6 +13,10 @@
 #include "base/ranges/algorithm.h"
 #include "base/stl_util.h"
 
+// This class is DEPRECATED. Instead please use one of:
+// - base::ScopedObservation for observing a single source.
+// - base::ScopedMultiSourceObservation for observing multiple sources.
+//
 // ScopedObserver is used to keep track of the set of sources an object has
 // attached itself to as an observer. When ScopedObserver is destroyed it
 // removes the object as an observer from all sources it has been added to.