Implement tracking of BUILD.gn files used to define target, toolchain or

tools/gn/toolchain.h

Index: tools/gn/toolchain.h
diff --git a/tools/gn/toolchain.h b/tools/gn/toolchain.h
index 7b61f349b9c6065b9e9f0ecd7fb9316e0a127e37..34b25838319dfa0fe9c0a2342e5b236fb058705f 100644
--- a/tools/gn/toolchain.h
+++ b/tools/gn/toolchain.h
@@ -75,7 +75,9 @@ class Toolchain : public Item {
   // Loader::GetToolchainSettings(). Many toolchain objects may be created in a
   // given build, but only a few might be used, and the Loader is in charge of
   // this process.
-  Toolchain(const Settings* settings, const Label& label);
+  Toolchain(const Settings* settings,
+            const Label& label,
+            const std::set<uint32_t>& source_files_hashes);
   ~Toolchain() override;
 
   // Item overrides.