Don't inherit source sets across static libs in GN.

tools/gn/target.h

Index: tools/gn/target.h
diff --git a/tools/gn/target.h b/tools/gn/target.h
index ea0997d3abac2c105cc3dc0b81ea44c49bcd3f5e..644704355c3ca3f82d08e293fd3afe97fc0f3320 100644
--- a/tools/gn/target.h
+++ b/tools/gn/target.h
@@ -198,8 +198,10 @@ class Target : public Item {
   bool external_;
 
   // Static libraries and source sets from transitive deps. These things need
-  // to be linked only with the end target (executable, shared library). These
-  // do not get pushed beyond shared library boundaries.
+  // to be linked only with the end target (executable, shared library). Source
+  // sets do not get pushed beyond static library boundaries, and neither
+  // source sets nor static libraries get pushed beyond sahred library
+  // boundaries.
   std::set<const Target*> inherited_libraries_;
 
   // These libs and dirs are inherited from statically linked deps and all