Revert of Move gn _run target generation into test().

extensions/BUILD.gn

Index: extensions/BUILD.gn
diff --git a/extensions/BUILD.gn b/extensions/BUILD.gn
index a624a4529ab99ec7ac76549811e806010b308bd2..e3600d8ff6e2a8ed6994d30f60e34cd7b19ae24a 100644
--- a/extensions/BUILD.gn
+++ b/extensions/BUILD.gn
@@ -138,6 +138,15 @@
     "//third_party/WebKit/public:resources",
     "//ui/resources",
     "//ui/strings",
+  ]
+}
+
+# TODO(GYP): Delete this after we've converted everything to GN.
+# The _run targets exist only for compatibility w/ GYP.
+group("extensions_unittests_run") {
+  testonly = true
+  deps = [
+    ":extensions_unittests",
   ]
 }
 
@@ -207,6 +216,15 @@
 
     deps += [ "//chromeos:test_support" ]
   }
+}
+
+# TODO(GYP): Delete this after we've converted everything to GN.
+# The _run targets exist only for compatibility w/ GYP.
+group("extensions_browsertests_run") {
+  testonly = true
+  deps = [
+    ":extensions_browsertests",
+  ]
 }
 
 test("extensions_browsertests") {