extensions: Add GN version of extensions_shell_and_test_pak and unittests target.

extensions/shell/BUILD.gn

Index: extensions/shell/BUILD.gn
diff --git a/ash/resources/BUILD.gn b/extensions/shell/BUILD.gn
similarity index 64%
copy from ash/resources/BUILD.gn
copy to extensions/shell/BUILD.gn
index 25f61ec6534b85cf8b956871bcee9ec80d5e7c6b..5e501e3c1bf4ccaec27017fb01fd32e9ff23bc92 100644
--- a/ash/resources/BUILD.gn
+++ b/extensions/shell/BUILD.gn
@@ -5,10 +5,9 @@
 import("//tools/grit/grit_rule.gni")
 
 grit("resources") {
-  source = "ash_resources.grd"
+  source = "app_shell_resources.grd"
   outputs = [
-    "grit/ash_resources.h",
-    "ash_resources_100_percent.pak",
-    "ash_resources_200_percent.pak",
+    "grit/app_shell_resources.h",
+    "app_shell_resources.pak",
   ]
 }