gn2bp: add srcs support to Target

Test: //components/cronet/android:cronet
Change-Id: I51f1258d741485cc85d5d7d7bbc5ea6f308b6c5a
diff --git a/tools/gn2bp/gen_android_bp b/tools/gn2bp/gen_android_bp
index f1a4a3b..e07be5c 100755
--- a/tools/gn2bp/gen_android_bp
+++ b/tools/gn2bp/gen_android_bp
@@ -199,6 +199,7 @@
 
   def __init__(self, name):
     self.name = name
+    self.srcs = set()
     self.shared_libs = set()
     self.static_libs = set()
     self.whole_static_libs = set()
@@ -209,6 +210,7 @@
 
   def to_string(self, output):
     nested_out = []
+    self._output_field(nested_out, 'srcs')
     self._output_field(nested_out, 'shared_libs')
     self._output_field(nested_out, 'static_libs')
     self._output_field(nested_out, 'whole_static_libs')