Remove ExtensionService::VerifyAllExtensions()

Refactor ExtensionService and InstallVerifier so that:
- InstallVerifier handles its own bootstrapping
- Requests to verify an extension (or all extensions) go to InstallVerifier
- InstallVerifier doesn't take callbacks for verifications

Also added an Observer class to ExtensionPrefs so that we can listen for
DisableReasons changing.

BUG=351891
BUG=328916

Review URL: https://codereview.chromium.org/200763002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@259863 0039d316-1c4b-4281-b951-d872f2087c98
diff --git a/chrome/browser/extensions/extension_system_impl.cc b/chrome/browser/extensions/extension_system_impl.cc
index 73c17a2..169b779 100644
--- a/chrome/browser/extensions/extension_system_impl.cc
+++ b/chrome/browser/extensions/extension_system_impl.cc
@@ -168,8 +168,8 @@
   // These services must be registered before the ExtensionService tries to
   // load any extensions.
   {
-    install_verifier_.reset(new InstallVerifier(ExtensionPrefs::Get(profile_),
-                                                profile_->GetRequestContext()));
+    install_verifier_.reset(
+        new InstallVerifier(ExtensionPrefs::Get(profile_), profile_));
     install_verifier_->Init();
 
     management_policy_.reset(new ManagementPolicy);