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/tests/WritePixelsTest.cpp b/tests/WritePixelsTest.cpp
index 1c5ce81..c38e4d6 100644
--- a/tests/WritePixelsTest.cpp
+++ b/tests/WritePixelsTest.cpp
@@ -382,7 +382,7 @@
                                                       rect.fLeft, rect.fTop));
 
                 // we should change the genID iff pixels were actually written.
-                SkIRect canvasRect = SkIRect::MakeSize(canvas.getDeviceSize());
+                SkIRect canvasRect = SkIRect::MakeSize(canvas.getBaseLayerSize());
                 SkIRect writeRect = SkIRect::MakeXYWH(rect.fLeft, rect.fTop,
                                                       bmp.width(), bmp.height());
                 bool intersects = SkIRect::Intersects(canvasRect, writeRect) ;