Fix unref order in SkGpuDevice

GrContext should be unrefed after GrDrawContext.

TBR=bsalomon@google.com
BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1569833004

Review URL: https://codereview.chromium.org/1569833004
diff --git a/src/gpu/SkGpuDevice.cpp b/src/gpu/SkGpuDevice.cpp
index 89273be..cd678af 100644
--- a/src/gpu/SkGpuDevice.cpp
+++ b/src/gpu/SkGpuDevice.cpp
@@ -10,7 +10,6 @@
 #include "GrBlurUtils.h"
 #include "GrContext.h"
 #include "SkDraw.h"
-#include "GrDrawContext.h"
 #include "GrGpu.h"
 #include "GrGpuResourcePriv.h"
 #include "GrImageIDTextureAdjuster.h"
@@ -171,13 +170,11 @@
 SkGpuDevice::SkGpuDevice(GrRenderTarget* rt, int width, int height,
                          const SkSurfaceProps* props, unsigned flags)
     : INHERITED(SkSurfacePropsCopyOrDefault(props))
-{
-    fContext = SkRef(rt->getContext());
+    , fContext(SkRef(rt->getContext()))
+    , fRenderTarget(SkRef(rt)) {
     fNeedClear = SkToBool(flags & kNeedClear_Flag);
     fOpaque = SkToBool(flags & kIsOpaque_Flag);
 
-    fRenderTarget = SkRef(rt);
-
     SkAlphaType at = fOpaque ? kOpaque_SkAlphaType : kPremul_SkAlphaType;
     SkImageInfo info = rt->surfacePriv().info(at).makeWH(width, height);
     SkPixelRef* pr = new SkGrPixelRef(info, rt);
@@ -226,11 +223,6 @@
     return texture->asRenderTarget();
 }
 
-SkGpuDevice::~SkGpuDevice() {
-    fRenderTarget->unref();
-    fContext->unref();
-}
-
 ///////////////////////////////////////////////////////////////////////////////
 
 bool SkGpuDevice::onReadPixels(const SkImageInfo& dstInfo, void* dstPixels, size_t dstRowBytes,
@@ -346,8 +338,7 @@
 
     SkASSERT(fRenderTarget != newRT);
 
-    fRenderTarget->unref();
-    fRenderTarget = newRT.detach();
+    fRenderTarget.reset(newRT.detach());
 
 #ifdef SK_DEBUG
     SkImageInfo info = fRenderTarget->surfacePriv().info(fOpaque ? kOpaque_SkAlphaType :