Use an infobar instead of alert box for extension install error.

chrome/browser/extensions/extension_install_ui.h

Index: chrome/browser/extensions/extension_install_ui.h
diff --git a/chrome/browser/extensions/extension_install_ui.h b/chrome/browser/extensions/extension_install_ui.h
index e5fabbef358580dc2c6238085345dd29e1b9e0e8..fe83e18e5ad4e4dbe0082fe3b802e93b15dbebd4 100644
--- a/chrome/browser/extensions/extension_install_ui.h
+++ b/chrome/browser/extensions/extension_install_ui.h
@@ -10,6 +10,7 @@
 
 #include "base/compiler_specific.h"
 #include "base/string16.h"
+#include "chrome/browser/extensions/crx_installer_error.h"
 
 class Browser;
 class SkBitmap;
@@ -31,7 +32,7 @@ class ExtensionInstallUI {
   virtual void OnInstallSuccess(const extensions::Extension* extension,
                                 SkBitmap* icon) = 0;
   // Called when an extension failed to install.
-  virtual void OnInstallFailure(const string16& error) = 0;
+  virtual void OnInstallFailure(const CrxInstallerError& error) = 0;
 
   // Whether or not to show the default UI after completing the installation.
   virtual void SetSkipPostInstallUI(bool skip_ui) = 0;