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/bench/WritePixelsBench.cpp b/bench/WritePixelsBench.cpp
index d9f8eb7..820bb59 100644
--- a/bench/WritePixelsBench.cpp
+++ b/bench/WritePixelsBench.cpp
@@ -46,7 +46,7 @@
     }
 
     void onDraw(int loops, SkCanvas* canvas) override {
-        SkISize size = canvas->getDeviceSize();
+        SkISize size = canvas->getBaseLayerSize();
 
         canvas->clear(0xFFFF0000);