Add an extension bubble explaining which extensions are in dev mode.

extensions/common/extension.cc

Index: extensions/common/extension.cc
diff --git a/extensions/common/extension.cc b/extensions/common/extension.cc
index 10e065b97f791fa7c270c748dd59f85714407db7..db9da4c4939413669324146919af14e6d16ff1d4 100644
--- a/extensions/common/extension.cc
+++ b/extensions/common/extension.cc
@@ -386,6 +386,11 @@ bool Extension::ShouldNotBeVisible() const {
   return false;
 }
 
+bool Extension::IsDevModeExtension() const {
+  return location() == Manifest::UNPACKED ||
+         location() == Manifest::COMMAND_LINE;
+}
+
 Extension::ManifestData* Extension::GetManifestData(const std::string& key)
     const {
   DCHECK(finished_parsing_manifest_ || thread_checker_.CalledOnValidThread());