hide trivial helpers on canvas

BUG=skia:

Change-Id: I42d4ca92897bde5bd8e0575a0104b12b83134cef
Reviewed-on: https://skia-review.googlesource.com/8852
Reviewed-by: Cary Clark <caryclark@google.com>
Commit-Queue: Mike Reed <reed@google.com>
diff --git a/samplecode/SampleManyRects.cpp b/samplecode/SampleManyRects.cpp
index 66274a4..cb19595 100644
--- a/samplecode/SampleManyRects.cpp
+++ b/samplecode/SampleManyRects.cpp
@@ -33,7 +33,7 @@
     }
 
     void onDrawContent(SkCanvas* canvas) override {
-        SkISize dsize = canvas->getDeviceSize();
+        SkISize dsize = canvas->getBaseLayerSize();
         canvas->clear(0xFFF0E0F0);
 
         for (int i = 0; i < N; ++i) {