Break the circular dependency in the new mojom bindings generation pipeline.

mojo/public/python/BUILD.gn

Index: mojo/public/python/BUILD.gn
diff --git a/mojo/public/python/BUILD.gn b/mojo/public/python/BUILD.gn
index 5fe296cea696941cfa563cad6e6172e6ec4f4372..c3c77bb4a54c852038a600cf5aedd5629a03497c 100644
--- a/mojo/public/python/BUILD.gn
+++ b/mojo/public/python/BUILD.gn
@@ -115,7 +115,14 @@ action("import_interface_bindings") {
   deps = mojom_deps
 }
 
-copy("bindings") {
+group("bindings") {

[rudominer, 2015/11/06 23:07:25]

This target is depended on by the "python" target. Previously that meant the
Python target was depending on the thing that is now called
"serialization_bindings". Do you know why that dependency was there before and
why it is not needed now?

[azani, 2015/11/06 23:39:00]

Sorry, this should depend on serialization_bindings.

+  deps = [

[rudominer, 2015/11/06 23:07:25]

should this target depend on :mojo_system_impl? Or should there be a comment
explaining any consumer of this target must depend on :mojo_system_impl?

[azani, 2015/11/06 23:39:00]

I don't know. I think that in practice, yes, you will always need
mojo_system_impl, but I don't understand what is going on enough to make that
change.

+    ":import_interface_bindings",
+  ]
+}
+
+# This target lists dependencies for mojom serialization only.

[rudominer, 2015/11/06 23:07:25]

Could you add one more sentence fleshing this out a little. Do you mean that
this target allows one to get Mojo serialization in Python without pulling in
the full Mojo application stack?

[azani, 2015/11/06 23:39:00]

Done.

+copy("serialization_bindings") {
   sources = [
     "mojo_bindings/__init__.py",
     "mojo_bindings/descriptor.py",
@@ -129,7 +136,6 @@ copy("bindings") {
     "$root_out_dir/python/mojo_bindings/{{source_file_part}}",
   ]
   deps = [
-    ":import_interface_bindings",
     ":mojo_system",
   ]
 }