Update from chromium https://crrev.com/301725/

base/scoped_observer.h

Index: base/scoped_observer.h
diff --git a/base/scoped_observer.h b/base/scoped_observer.h
index 3754ed57e1d27c3b98f59b8c41262c7d80de9507..5b0d53353bab913125335acc0d925987c60b7dae 100644
--- a/base/scoped_observer.h
+++ b/base/scoped_observer.h
@@ -9,6 +9,7 @@
 #include <vector>
 
 #include "base/basictypes.h"
+#include "base/logging.h"
 
 // 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
@@ -30,7 +31,9 @@ class ScopedObserver {
 
   // Remove the object passed to the constructor as an observer from |source|.
   void Remove(Source* source) {
-    sources_.erase(std::find(sources_.begin(), sources_.end(), source));
+    auto it = std::find(sources_.begin(), sources_.end(), source);
+    DCHECK(it != sources_.end());
+    sources_.erase(it);
     source->RemoveObserver(observer_);
   }