Re-land sksl fragment processor support

This reverts commit ed50200682e0de72c3abecaa4d5324ebcd1ed9f9.

Bug: skia:
Change-Id: I9caa7454b391450620d6989dc472abb3cf7a2cab
Reviewed-on: https://skia-review.googlesource.com/20965
Reviewed-by: Ben Wagner <benjaminwagner@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
diff --git a/src/sksl/ir/SkSLSetting.cpp b/src/sksl/ir/SkSLSetting.cpp
new file mode 100644
index 0000000..2d4a8ba
--- /dev/null
+++ b/src/sksl/ir/SkSLSetting.cpp
@@ -0,0 +1,22 @@
+/*
+ * Copyright 2017 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "SkSLSetting.h"
+#include "SkSLIRGenerator.h"
+#include "SkSLVariableReference.h"
+
+namespace SkSL {
+
+std::unique_ptr<Expression> Setting::constantPropagate(const IRGenerator& irGenerator,
+                                                       const DefinitionMap& definitions) {
+        if (irGenerator.fSettings->fReplaceSettings) {
+            return VariableReference::copy_constant(irGenerator, fValue.get());
+        }
+        return nullptr;
+    }
+} // namespace
+