add SkCanvas constructor that explicitly creates no_config with dimensions

BUG=skia:
R=scroggo@google.com, halcanary@google.com

Author: reed@google.com

Review URL: https://codereview.chromium.org/137833016

git-svn-id: http://skia.googlecode.com/svn/trunk@13272 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/src/utils/SkNWayCanvas.cpp b/src/utils/SkNWayCanvas.cpp
index da8bdb6..831d7bf 100644
--- a/src/utils/SkNWayCanvas.cpp
+++ b/src/utils/SkNWayCanvas.cpp
@@ -7,14 +7,8 @@
  */
 #include "SkNWayCanvas.h"
 
-static SkBitmap make_noconfig_bm(int width, int height) {
-    SkBitmap bm;
-    bm.setConfig(SkBitmap::kNo_Config, width, height);
-    return bm;
-}
-
 SkNWayCanvas::SkNWayCanvas(int width, int height)
-        : INHERITED(make_noconfig_bm(width, height)) {}
+        : INHERITED(width, height) {}
 
 SkNWayCanvas::~SkNWayCanvas() {
     this->removeAll();