gn2bp: fix proto parsing

GnUtils and gen_android_bp already have support for proto targets.
It looks pretty generic and quite a bit more evolved than what we have
done so far.

Test: //net:net
Change-Id: I1ca0d004650b4dc5e09fae4a0f4159699b4de96b
diff --git a/tools/gn2bp/gn_utils.py b/tools/gn2bp/gn_utils.py
index 1aa352e..589b12a 100644
--- a/tools/gn2bp/gn_utils.py
+++ b/tools/gn2bp/gn_utils.py
@@ -353,10 +353,10 @@
     gen_desc = self.gn_desc_.get('%s_gen%s' % (name, toolchain))
     if gen_desc is None or gen_desc['type'] != 'action':
       return None, None
-    args = gen_desc.get('args', [])
-    if '/protoc' not in args[0]:
+    if gen_desc['script'] != '//tools/protoc_wrapper/protoc_wrapper.py':
       return None, None
     plugin = 'proto'
+    args = gen_desc.get('args', [])
     for arg in (arg for arg in args if arg.startswith('--plugin=')):
       # |arg| at this point looks like:
       #  --plugin=protoc-gen-plugin=gcc_like_host/protozero_plugin