Mark mini_installer as chromium_code in gyp builds.

chrome/installer/mini_installer/mini_installer.cc

Index: chrome/installer/mini_installer/mini_installer.cc
diff --git a/chrome/installer/mini_installer/mini_installer.cc b/chrome/installer/mini_installer/mini_installer.cc
index 7ce58263c4eb39afec816e9e95d5358a0b8bee1d..f3d7260dd40a03f4d2b8777238a181921e820f4d 100644
--- a/chrome/installer/mini_installer/mini_installer.cc
+++ b/chrome/installer/mini_installer/mini_installer.cc
@@ -781,7 +781,7 @@ void DeleteOldChromeTempDirectories() {
   if (!len || len >= temp.capacity())
     return;
 
-  for (int i = 0; i < _countof(kDirectoryPrefixes); ++i) {
+  for (size_t i = 0; i < _countof(kDirectoryPrefixes); ++i) {
     DeleteDirectoriesWithPrefix(temp.get(), kDirectoryPrefixes[i]);
   }
 }