Let mutators use ctx.AddNinjaFileDeps

Propagate extra ninja file deps through mutators so that they
can use ctx.AddNinjaFileDeps.

Test: blueprint tests
Change-Id: I299a0665c3f63b020ae345889fd78b91b91b215a
diff --git a/context_test.go b/context_test.go
index 6070c99..b9fb913 100644
--- a/context_test.go
+++ b/context_test.go
@@ -105,7 +105,7 @@
 		t.FailNow()
 	}
 
-	errs = ctx.ResolveDependencies(nil)
+	_, errs = ctx.ResolveDependencies(nil)
 	if len(errs) > 0 {
 		t.Errorf("unexpected dep errors:")
 		for _, err := range errs {
@@ -170,7 +170,7 @@
 		t.FailNow()
 	}
 
-	errs = ctx.ResolveDependencies(nil)
+	_, errs = ctx.ResolveDependencies(nil)
 	if len(errs) > 0 {
 		t.Errorf("unexpected dep errors:")
 		for _, err := range errs {
@@ -228,7 +228,7 @@
 		t.FailNow()
 	}
 
-	errs = ctx.ResolveDependencies(nil)
+	_, errs = ctx.ResolveDependencies(nil)
 	if len(errs) > 0 {
 		t.Errorf("unexpected dep errors:")
 		for _, err := range errs {