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/SampleChart.cpp b/samplecode/SampleChart.cpp
index 0c8949c..46ce7d3 100644
--- a/samplecode/SampleChart.cpp
+++ b/samplecode/SampleChart.cpp
@@ -99,8 +99,8 @@
 
     void onDrawContent(SkCanvas* canvas) override {
         bool sizeChanged = false;
-        if (canvas->getDeviceSize() != fSize) {
-            fSize = canvas->getDeviceSize();
+        if (canvas->getBaseLayerSize() != fSize) {
+            fSize = canvas->getBaseLayerSize();
             sizeChanged = true;
         }