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

tools/gn/toolchain.cc

Index: tools/gn/toolchain.cc
diff --git a/tools/gn/toolchain.cc b/tools/gn/toolchain.cc
index 70c6ce06f109bdcef256cd34b7372447f4431c95..9ec6932101fbfd2f584df6f0d826d612a2038913 100644
--- a/tools/gn/toolchain.cc
+++ b/tools/gn/toolchain.cc
@@ -27,10 +27,10 @@ const char* Toolchain::kToolCopy = "copy";
 const char* Toolchain::kToolCopyBundleData = "copy_bundle_data";
 const char* Toolchain::kToolCompileXCAssets = "compile_xcassets";
 
-Toolchain::Toolchain(const Settings* settings, const Label& label)
-    : Item(settings, label),
-      setup_complete_(false) {
-}
+Toolchain::Toolchain(const Settings* settings,
+                     const Label& label,
+                     const std::set<uint32_t>& source_files_hashes)
+    : Item(settings, label, source_files_hashes), setup_complete_(false) {}
 
 Toolchain::~Toolchain() {
 }