Allow relative paths to external extension files for some providers, error out for others.

chrome/browser/extensions/external_extension_loader.h

Index: chrome/browser/extensions/external_extension_loader.h
diff --git a/chrome/browser/extensions/external_extension_loader.h b/chrome/browser/extensions/external_extension_loader.h
index 594fb9e75c072867db449faf2334e9587a83b61b..f4864cbbe55fd72dab7e1b94ab548dfe5e8a0992 100644
--- a/chrome/browser/extensions/external_extension_loader.h
+++ b/chrome/browser/extensions/external_extension_loader.h
@@ -6,6 +6,7 @@
 #define CHROME_BROWSER_EXTENSIONS_EXTERNAL_EXTENSION_LOADER_H_
 #pragma once
 
+#include "base/file_path.h"
 #include "base/ref_counted.h"
 #include "base/scoped_ptr.h"
 
@@ -41,6 +42,13 @@ class ExternalExtensionLoader
   // in prefs_ and then call LoadFinished.
   virtual void StartLoading() = 0;
 
+  // Some external providers allow relative file paths to local CRX files.
+  // Subclasses that want this behavior should override this method to
+  // return the absolute path from which relative paths should be resolved.
+  // By default, return an empty path, which indicates that relative paths
+  // are not allowed.
+  virtual const FilePath GetBaseCrxFilePath();
+
  protected:
   virtual ~ExternalExtensionLoader() {}