Make SkGpuDevice constructors private.

R=robertphillips@google.com

Author: bsalomon@google.com

Review URL: https://codereview.chromium.org/517033002
diff --git a/samplecode/SampleApp.cpp b/samplecode/SampleApp.cpp
index 29233d0..34d1b97 100644
--- a/samplecode/SampleApp.cpp
+++ b/samplecode/SampleApp.cpp
@@ -275,7 +275,7 @@
                                    SampleWindow* win) {
 #if SK_SUPPORT_GPU
         if (IsGpuDeviceType(dType) && NULL != fCurContext) {
-            SkAutoTUnref<SkBaseDevice> device(new SkGpuDevice(fCurContext, fCurRenderTarget));
+            SkAutoTUnref<SkBaseDevice> device(SkGpuDevice::Create(fCurRenderTarget));
             return new SkCanvas(device);
         } else
 #endif