Factor GrTexture into public GrTexture and private GrTextureImpl.

bench/GrResourceCacheBench.cpp

Index: bench/GrResourceCacheBench.cpp
diff --git a/bench/GrResourceCacheBench.cpp b/bench/GrResourceCacheBench.cpp
index 6767ca1d49ce66006dadcbbecb6719e56efa52dd..b2a5d1e7182197dd11c4fba3f7964c77fbad8063 100644
--- a/bench/GrResourceCacheBench.cpp
+++ b/bench/GrResourceCacheBench.cpp
@@ -62,7 +62,7 @@ public:
     }
 
     static GrResourceKey ComputeKey(const GrTextureDesc& desc) {
-        return GrTexture::ComputeScratchKey(desc);
+        return GrTextureImpl::ComputeScratchKey(desc);
     }
 
     int fID;