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/XfermodeBench.cpp b/bench/XfermodeBench.cpp
index 605d167..b12e458 100644
--- a/bench/XfermodeBench.cpp
+++ b/bench/XfermodeBench.cpp
@@ -26,7 +26,7 @@
     void onDraw(int loops, SkCanvas* canvas) override {
         const char* text = "Hamburgefons";
         size_t len = strlen(text);
-        SkISize size = canvas->getDeviceSize();
+        SkISize size = canvas->getBaseLayerSize();
         SkRandom random;
         for (int i = 0; i < loops; ++i) {
             SkPaint paint;