Remove image usage type enum. Use GrTextureParams instead.

BUG=skia:

Review URL: https://codereview.chromium.org/1404433002
diff --git a/src/image/SkImage_Gpu.h b/src/image/SkImage_Gpu.h
index 8bfdfa7..530c044 100644
--- a/src/image/SkImage_Gpu.h
+++ b/src/image/SkImage_Gpu.h
@@ -36,7 +36,7 @@
     }
 
     bool getROPixels(SkBitmap*) const override;
-    GrTexture* asTextureRef(GrContext* ctx, SkImageUsageType usage) const override;
+    GrTexture* asTextureRef(GrContext* ctx, const GrTextureParams& params) const override;
     SkImage* onNewSubset(const SkIRect&) const override;
 
     GrTexture* peekTexture() const override { return fTexture; }