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/gm/convexpolyclip.cpp b/gm/convexpolyclip.cpp
index 5a8844a..bd945dd 100644
--- a/gm/convexpolyclip.cpp
+++ b/gm/convexpolyclip.cpp
@@ -143,7 +143,7 @@
 
         SkPaint bgPaint;
         bgPaint.setAlpha(0x15);
-        SkISize size = canvas->getDeviceSize();
+        SkISize size = canvas->getBaseLayerSize();
         canvas->drawBitmapRect(fBmp, SkRect::MakeIWH(size.fWidth, size.fHeight), &bgPaint);
 
         constexpr char kTxt[] = "Clip Me!";