GN: Add ability to specify a depfile for custom targets.

tools/gn/script_target_generator.cc

Index: tools/gn/script_target_generator.cc
diff --git a/tools/gn/script_target_generator.cc b/tools/gn/script_target_generator.cc
index c9616950e4b48ce4ca90f2942e7f4e27b45eec24..272061c488be588e66ae98043c974c6ff80c323f 100644
--- a/tools/gn/script_target_generator.cc
+++ b/tools/gn/script_target_generator.cc
@@ -4,6 +4,7 @@
 
 #include "tools/gn/script_target_generator.h"
 
+#include "tools/gn/build_settings.h"
 #include "tools/gn/err.h"
 #include "tools/gn/filesystem_utils.h"
 #include "tools/gn/scope.h"
@@ -48,6 +49,10 @@ void ScriptTargetGenerator::DoRun() {
   if (err_->has_error())
     return;
 
+  FillDepfile();
+  if (err_->has_error())
+    return;
+
   // Script outputs don't depend on the current toolchain so we can skip adding
   // that dependency.
 }
@@ -77,3 +82,12 @@ void ScriptTargetGenerator::FillScriptArgs() {
     return;
   target_->script_values().swap_in_args(&args);
 }
+
+void ScriptTargetGenerator::FillDepfile() {
+  const Value* value = scope_->GetValue(variables::kDepfile, true);
+  if (!value)
+    return;
+  target_->script_values().set_depfile(
+      scope_->settings()->build_settings()->build_dir().ResolveRelativeFile(
+          value->string_value()));
+}