Make SkBlitter hierarchy explicit about what needs to be implemented.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2053823002

Review-Url: https://codereview.chromium.org/2053823002
diff --git a/src/core/SkLinearBitmapPipeline_sample.h b/src/core/SkLinearBitmapPipeline_sample.h
index 930759f..5184a41 100644
--- a/src/core/SkLinearBitmapPipeline_sample.h
+++ b/src/core/SkLinearBitmapPipeline_sample.h
@@ -428,6 +428,7 @@
     PixelAccessor<colorType, colorProfile> fStrategy;
 };
 
+// -- BilerpSampler --------------------------------------------------------------------------------
 // BilerpSampler - use a bilerp filter to create runs of destination pixels.
 template<SkColorType colorType, SkColorProfileType colorProfile, typename Next>
 class BilerpSampler : public SkLinearBitmapPipeline::SampleProcessorInterface {