Add Sk prefix to CreateBitmapShader.

It's neither static nor namespaced, so we should probably prefix it.

BUG=skia:

Review URL: https://codereview.chromium.org/754083002
diff --git a/src/core/SkBitmapProcShader.h b/src/core/SkBitmapProcShader.h
index cd52d99..17bb83b 100644
--- a/src/core/SkBitmapProcShader.h
+++ b/src/core/SkBitmapProcShader.h
@@ -78,7 +78,7 @@
 
 // If alloc is non-NULL, it will be used to allocate the returned SkShader, and MUST outlive
 // the SkShader.
-SkShader* CreateBitmapShader(const SkBitmap& src, SkShader::TileMode, SkShader::TileMode,
-                             const SkMatrix* localMatrix, SkTBlitterAllocator* alloc);
+SkShader* SkCreateBitmapShader(const SkBitmap& src, SkShader::TileMode, SkShader::TileMode,
+                               const SkMatrix* localMatrix, SkTBlitterAllocator* alloc);
 
 #endif