base::Scoped*Observation; DCHECK nullptr observation queries.

Bug: 1145565
Change-Id: Id4cc4569c4eca64cc250e11325f921a873f3819d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2547048Reviewed-by: François Doray <fdoray@chromium.org>
Commit-Queue: François Doray <fdoray@chromium.org>
Commit-Queue: Sigurður Ásgeirsson <siggi@chromium.org>
Auto-Submit: Sigurður Ásgeirsson <siggi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828990}

base/scoped_multi_source_observation.h

@@ -86,6 +86,7 @@ class ScopedMultiSourceObservation {
 
   // Returns true if |source| is being observed.
   bool IsObservingSource(Source* source) const {
+    DCHECK(source);
     return base::Contains(sources_, source);
   }
 

base/scoped_observation.h

@@ -75,7 +75,10 @@ class ScopedObservation {
   bool IsObserving() const { return source_ != nullptr; }
 
   // Returns true if |source| is being observed.
-  bool IsObservingSource(Source* source) const { return source_ == source; }
+  bool IsObservingSource(Source* source) const {
+    DCHECK(source);
+    return source_ == source;
+  }
 
  private:
   Observer* const observer_;