Loosen up assert in SkImage_Gpu::asTextureProxyRef

In the DDL world, the actual constraint is that the contexts have
the same ID. The old version was catching the case where an already
instantiated SkImage_Gpu was being drawn into a DDL.

Bug: 882457
Change-Id: I09ac0d5e176e8de5f5089d11d7495766c92f2450
Reviewed-on: https://skia-review.googlesource.com/153665
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>

src/image/SkImage_Gpu.cpp

@@ -123,7 +123,7 @@ sk_sp<GrTextureProxy> SkImage_Gpu::asTextureProxyRef(GrContext* context,
                                                      SkColorSpace* dstColorSpace,
                                                      sk_sp<SkColorSpace>* texColorSpace,
                                                      SkScalar scaleAdjust[2]) const {
-    if (context != fContext.get()) {
+    if (context->uniqueID() != fContext->uniqueID()) {
         SkASSERT(0);
         return nullptr;
     }