Begin work on extension shelf for Linux.

chrome/browser/extensions/extension_host.cc

Index: chrome/browser/extensions/extension_host.cc
diff --git a/chrome/browser/extensions/extension_host.cc b/chrome/browser/extensions/extension_host.cc
index dd46e072b2c066e62a234918ddada45c2be0dbfa..21acf4f1f39f16e26cabcfff4e4ded33d0b90659 100644
--- a/chrome/browser/extensions/extension_host.cc
+++ b/chrome/browser/extensions/extension_host.cc
@@ -108,6 +108,8 @@ void ExtensionHost::CreateView(Browser* browser) {
   // We own |view_|, so don't auto delete when it's removed from the view
   // hierarchy.
   view_->SetParentOwned(false);
+#elif defined(OS_LINUX)
+  // FIXME(phajdan.jr): Add extension view for GTK.
 #else
   // TODO(port)
   NOTREACHED();