Ignore ctm param of SkShader::onAppendStages()

Clang 3.8.0:

../../include/core/SkShader.h:513:49: error: unused parameter 'ctm' [-Werror,-Wunused-parameter]
                                const SkMatrix& ctm) const {


R=mtklein@google.com

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4867

Change-Id: I242180cb33ad099b28b06654934029690da195ee
Reviewed-on: https://skia-review.googlesource.com/4867
Commit-Queue: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Mike Klein <mtklein@chromium.org>
diff --git a/include/core/SkShader.h b/include/core/SkShader.h
index d873ad1..45302cc 100644
--- a/include/core/SkShader.h
+++ b/include/core/SkShader.h
@@ -510,7 +510,7 @@
     }
 
     virtual bool onAppendStages(SkRasterPipeline*, SkColorSpace*, SkFallbackAlloc*,
-                                const SkMatrix& ctm) const {
+                                const SkMatrix&) const {
         return false;
     }