Tighten up GrSurfaceProxy typing

This may reduce the number of "why not GrTextureProxy" issues

Change-Id: I9e0e5042f5801ba9a933b697a380cb0cb54b4522
Reviewed-on: https://skia-review.googlesource.com/8510
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
diff --git a/src/core/SkSpecialImage.h b/src/core/SkSpecialImage.h
index bf168f0..1c9c5a1 100644
--- a/src/core/SkSpecialImage.h
+++ b/src/core/SkSpecialImage.h
@@ -89,7 +89,7 @@
     static sk_sp<SkSpecialImage> MakeDeferredFromGpu(GrContext*,
                                                      const SkIRect& subset,
                                                      uint32_t uniqueID,
-                                                     sk_sp<GrSurfaceProxy>,
+                                                     sk_sp<GrTextureProxy>,
                                                      sk_sp<SkColorSpace>,
                                                      const SkSurfaceProps* = nullptr,
                                                      SkAlphaType at = kPremul_SkAlphaType);