ScopedObservation: CHECK on double observation.

Because base::ScopedObservation is strictly a singular observation,
it has exposed at least one latent release blocker bug on replacing
ScopedObserver. Unfortunately this manifested as a use-after-free, as
there was no test or build with DCHECK_IS_ON that covered this case.
See https://crbug.com/1153961#c14 for the details.

Bug: 1145565
Change-Id: Ibe198ca413e94676fa5dac3b94837b9999b3cb59
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2627763Reviewed-by: François Doray <fdoray@chromium.org>
Commit-Queue: Sigurður Ásgeirsson <siggi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843641}

base/scoped_observation.h

@@ -56,7 +56,9 @@ class ScopedObservation {
   // Adds the object passed to the constructor as an observer on |source|.
   // IsObserving() must be false.
   void Observe(Source* source) {
-    DCHECK_EQ(source_, nullptr);
+    // TODO(https://crbug.com/1145565): Make this a DCHECK once ScopedObserver
+    //     has been fully retired.
+    CHECK_EQ(source_, nullptr);
     source_ = source;
     (source_->*AddObsFn)(observer_);
   }