Allow retrieval of media device ID salt even after ResourceContext has gone away.

content/public/browser/resource_context.h

Index: content/public/browser/resource_context.h
diff --git a/content/public/browser/resource_context.h b/content/public/browser/resource_context.h
index 1308d7657b9136d3c4a36a73ffb22ba4afd2dda5..36b745e58bbf1e3a8ba98749786d6cfde9157dc1 100644
--- a/content/public/browser/resource_context.h
+++ b/content/public/browser/resource_context.h
@@ -68,11 +68,19 @@ class CONTENT_EXPORT ResourceContext : public base::SupportsUserData {
   // resource metadata.
   virtual bool AllowCameraAccess(const GURL& origin) = 0;
 
-  // Returns a random salt string that is used for creating media device IDs.
-  // The salt should be stored in the current user profile and should be reset
-  // if cookies are cleared. Returns an empty string per default.
-  // TODO(perkj): Make this method pure virtual when crbug/315022 is fixed.
-  virtual std::string GetMediaDeviceIDSalt();
+  // Returns a callback that can be invoked to get a random salt
+  // string that is used for creating media device IDs.  The salt
+  // should be stored in the current user profile and should be reset
+  // if cookies are cleared. The default is an empty string.
+  //
+  // It is safe to hold on to the callback returned and use it without
+  // regard to the lifetime of ResourceContext, although in general
+  // you should not use it long after the profile has been destroyed.
+  //
+  // TODO(perkj): Make this method pure virtual when crbug/315022 is
+  // fixed.
+  typedef base::Callback<std::string()> SaltCallback;
+  virtual SaltCallback GetMediaDeviceIDSalt();
 };
 
 }  // namespace content