[Extensions] Validate file existence for chrome url overrides

extensions/common/manifest_constants.cc

Index: extensions/common/manifest_constants.cc
diff --git a/extensions/common/manifest_constants.cc b/extensions/common/manifest_constants.cc
index 51b5570889cebfd86f4a3d0c54f99d5d0a710fdc..1c3f94c608b3df71d17ba2ad2d817c7e5d431197 100644
--- a/extensions/common/manifest_constants.cc
+++ b/extensions/common/manifest_constants.cc
@@ -293,6 +293,7 @@ const char kChromeVersionTooLow[] =
 const char kDisabledByPolicy[] =
     "This extension has been disabled by your administrator.";
 const char kExpectString[] = "Expect string value.";
+const char kFileNotFound[] = "File not found: *.";
 const char kInvalidAboutPage[] = "Invalid value for 'about_page'.";
 const char kInvalidAboutPageExpectRelativePath[] =
     "Invalid value for 'about_page'. Value must be a relative path.";