[Extensions] Fixed removing from blacklist.

1. Moved SetExtensionBlacklisted to private section and replace all
occurrences to SetExtensionBlacklistState.
2. Added ExtensionServiceTest.RemoveExtensionFromBlacklist test.

BUG=

Review-Url: https://codereview.chromium.org/2147923002
Cr-Commit-Position: refs/heads/master@{#406509}
diff --git a/chrome/browser/extensions/extension_service.cc b/chrome/browser/extensions/extension_service.cc
index bb578e1..f0b864a 100644
--- a/chrome/browser/extensions/extension_service.cc
+++ b/chrome/browser/extensions/extension_service.cc
@@ -2359,7 +2359,8 @@
       continue;
     }
     registry_->RemoveBlacklisted(*it);
-    extension_prefs_->SetExtensionBlacklisted(extension->id(), false);
+    extension_prefs_->SetExtensionBlacklistState(extension->id(),
+                                                 extensions::NOT_BLACKLISTED);
     AddExtension(extension.get());
     UMA_HISTOGRAM_ENUMERATION("ExtensionBlacklist.UnblacklistInstalled",
                               extension->location(),