Extract InstallVerifier from ExtensionSystem.

chrome/browser/extensions/install_verifier.cc

Index: chrome/browser/extensions/install_verifier.cc
diff --git a/chrome/browser/extensions/install_verifier.cc b/chrome/browser/extensions/install_verifier.cc
index 3637ff77325500c1cc4f373000f49a74de80f472..073964779ba1de0e6362a4b8dff893cdeea2b1cd 100644
--- a/chrome/browser/extensions/install_verifier.cc
+++ b/chrome/browser/extensions/install_verifier.cc
@@ -17,6 +17,7 @@
 #include "chrome/browser/extensions/extension_management.h"
 #include "chrome/browser/extensions/extension_service.h"
 #include "chrome/browser/extensions/install_signer.h"
+#include "chrome/browser/extensions/install_verifier_factory.h"
 #include "chrome/common/chrome_switches.h"
 #include "chrome/grit/generated_resources.h"
 #include "content/public/browser/browser_context.h"
@@ -180,6 +181,12 @@ InstallVerifier::InstallVerifier(ExtensionPrefs* prefs,
 InstallVerifier::~InstallVerifier() {}
 
 // static
+InstallVerifier* InstallVerifier::Get(
+    content::BrowserContext* browser_context) {
+  return InstallVerifierFactory::GetForBrowserContext(browser_context);
+}
+
+// static
 bool InstallVerifier::ShouldEnforce() {
   return GetStatus() >= ENFORCE;
 }