Enable embedders to include Perfetto protos

This meant:
* Generating pbzero and pblite in the same paths,
  to avoid search path magic or having to specify
  "pbzero" or "pblite" in the include path.
* (From Primiano) Prevent generation of all Perfetto .py proto files to avoid naming conflicts.
* Adding the root of the perfetto generated files to the search path to
  let Perfetto protos include headers from other protos.


Change-Id: Ie90cbbf6cf8233d94abd324e3c07f15a0987d047
diff --git a/gn/ipc_library.gni b/gn/ipc_library.gni
index db4d72c..c555dd4 100644
--- a/gn/ipc_library.gni
+++ b/gn/ipc_library.gni
@@ -25,6 +25,7 @@
   proto_library(target_name) {
     perfetto_root_path = invoker.perfetto_root_path
 
+    generate_python = false
     generator_plugin_label =
         perfetto_root_path + "src/ipc/protoc_plugin:ipc_plugin"
     generator_plugin_suffix = ".ipc"
@@ -35,7 +36,7 @@
       deps += invoker.deps
     }
 
-    proto_out_dir = "protos_lite"
+    proto_out_dir = perfetto_root_path
     forward_variables_from(invoker,
                            [
                              "defines",