Extract InstallVerifier from ExtensionSystem.

chrome/browser/extensions/install_verifier.h

Index: chrome/browser/extensions/install_verifier.h
diff --git a/chrome/browser/extensions/install_verifier.h b/chrome/browser/extensions/install_verifier.h
index b5a922502ad647d6e6ffe9522e4433c3fac3e329..3a29276d08833183f695e8cb31c3f6294ac33cd9 100644
--- a/chrome/browser/extensions/install_verifier.h
+++ b/chrome/browser/extensions/install_verifier.h
@@ -14,6 +14,7 @@
 #include "base/memory/linked_ptr.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/memory/weak_ptr.h"
+#include "components/keyed_service/core/keyed_service.h"
 #include "extensions/browser/management_policy.h"
 #include "extensions/common/extension.h"
 
@@ -40,11 +41,15 @@ struct InstallSignature;
 //
 // This class should be kept notified of runtime changes to the set of
 // extensions installed from the webstore.
-class InstallVerifier : public ManagementPolicy::Provider {
+class InstallVerifier : public KeyedService,
+                        public ManagementPolicy::Provider {
  public:
   InstallVerifier(ExtensionPrefs* prefs, content::BrowserContext* context);
   ~InstallVerifier() override;
 
+  // Convenience method to return the InstallVerifier for a given |context|.
+  static InstallVerifier* Get(content::BrowserContext* context);
+
   // Returns whether install verification should be enforced.
   static bool ShouldEnforce();