Make sure SkDrawLooper objects can only be allocated on the heap.

Make constructors of SkLayerDrawLooper and SkBlurDrawLooper non-public.
Remove addLayer* methods from SkLayerDrawLooper. SkLayerDrawLooper::Builder is
used to create new objects.
Provide factory method for creating SkBlurDrawLooper.

BUG=2141
R=scroggo@google.com, reed@google.com, djsollen@google.com

Author: dominikg@chromium.org

Review URL: https://codereview.chromium.org/232913003

git-svn-id: http://skia.googlecode.com/svn/trunk@14200 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/samplecode/SampleTextOnPath.cpp b/samplecode/SampleTextOnPath.cpp
index f2b9af6..6d79e20 100644
--- a/samplecode/SampleTextOnPath.cpp
+++ b/samplecode/SampleTextOnPath.cpp
@@ -35,10 +35,10 @@
 
     canvas->drawPath(path, paint);
 
-    paint.setLooper(new SkBlurDrawLooper(SK_ColorBLACK,
-                                         SkBlurMask::ConvertRadiusToSigma(0.002f),
-                                         0.0f,
-                                         0.0f))->unref();
+    paint.setLooper(SkBlurDrawLooper::Create(SK_ColorBLACK,
+                                             SkBlurMask::ConvertRadiusToSigma(0.002f),
+                                             0.0f,
+                                             0.0f))->unref();
 
     const char* text = "DRAWING STROKED TEXT WITH A BLUR ON A PATH";
     size_t      len = strlen(text);