make SkDevice constructors explicit between offscreen and on/direct

http://codereview.appspot.com/4632044/



git-svn-id: http://skia.googlecode.com/svn/trunk@1620 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/bench/benchmain.cpp b/bench/benchmain.cpp
index 7fce807..29963a8 100644
--- a/bench/benchmain.cpp
+++ b/bench/benchmain.cpp
@@ -171,10 +171,10 @@
         case kRaster_Backend:
             bitmap.allocPixels();
             erase(bitmap);
-            device = new SkDevice(NULL, bitmap, true);
+            device = new SkDevice(bitmap);
             break;
         case kGPU_Backend:
-            device = new SkGpuDevice(context, bitmap, SkGpuDevice::Current3DApiRenderTarget());
+            device = new SkGpuDevice(context, SkGpuDevice::Current3DApiRenderTarget());
 //            device->clear(0xFFFFFFFF);
             break;
         case kPDF_Backend: