Add a group policy controlling which sites can install extensions.

chrome/browser/extensions/crx_installer.h

Index: chrome/browser/extensions/crx_installer.h
diff --git a/chrome/browser/extensions/crx_installer.h b/chrome/browser/extensions/crx_installer.h
index 5a9d51b4d25a8d51fbb3df11d9b5f56c86b5cc1e..e33b8ef63e3dec0196d707eec4cde57d68a57d2b 100644
--- a/chrome/browser/extensions/crx_installer.h
+++ b/chrome/browser/extensions/crx_installer.h
@@ -54,6 +54,15 @@ class CrxInstaller
     : public SandboxedExtensionUnpackerClient,
       public ExtensionInstallUI::Delegate {
  public:
+  // Used in histograms; do not change order.
+  enum OffStoreInstallAllowReason {
+    OffStoreInstallDisallowed,
+    OffStoreInstallAllowedFromSettingsPage,
+    OffStoreInstallAllowedBecausePref,

[Mattias Nissler (ping if slow), 2012/06/08 09:38:22]

nit: I'd rename this to OffStoreInstallAllowedByPref, up to you.

+    OffStoreInstallAllowedInTest,
+    NumOffStoreInstallAllowReasons

[Mattias Nissler (ping if slow), 2012/06/08 09:38:22]

Style guide says "Though the Google C++ Style Guide says to use kConstantNaming
for enums, Chrome still uses MACRO_STYLE naming for enums for consistency with
our existing code.". These enum constants don't follow either style.

+  };
+
   // Extensions will be installed into frontend->install_directory(),
   // then registered with |frontend|. Any install UI will be displayed
   // using |client|. Pass NULL for |client| for silent install
@@ -146,8 +155,12 @@ class CrxInstaller
     install_cause_ = install_cause;
   }
 
-  bool allow_off_store_install() const { return allow_off_store_install_; }
-  void set_allow_off_store_install(bool val) { allow_off_store_install_ = val; }
+  OffStoreInstallAllowReason off_store_install_allow_reason() const {
+    return off_store_install_allow_reason_;
+  }
+  void set_off_store_install_allow_reason(OffStoreInstallAllowReason reason) {
+    off_store_install_allow_reason_ = reason;
+  }
 
   void set_page_ordinal(const StringOrdinal& page_ordinal) {
     page_ordinal_ = page_ordinal;
@@ -312,7 +325,7 @@ class CrxInstaller
   int creation_flags_;
 
   // Whether to allow off store installation.
-  bool allow_off_store_install_;
+  OffStoreInstallAllowReason off_store_install_allow_reason_;
 
   DISALLOW_COPY_AND_ASSIGN(CrxInstaller);
 };