Remove image usage type enum. Use GrTextureParams instead.

BUG=skia:

Review URL: https://codereview.chromium.org/1404433002
diff --git a/src/utils/SkImageGeneratorUtils.cpp b/src/utils/SkImageGeneratorUtils.cpp
index 42a0fe5..8202dbe 100644
--- a/src/utils/SkImageGeneratorUtils.cpp
+++ b/src/utils/SkImageGeneratorUtils.cpp
@@ -56,7 +56,8 @@
     {}
 
 protected:
-    GrTexture* onGenerateTexture(GrContext* ctx, SkImageUsageType, const SkIRect* subset) override {
+    GrTexture* onGenerateTexture(GrContext* ctx, const GrTextureParams&,
+                                 const SkIRect* subset) override {
         if (ctx) {
             SkASSERT(ctx == fCtx.get());
         }
@@ -112,7 +113,8 @@
         return fImage->readPixels(info, pixels, rowBytes, 0, 0);
     }
 
-    GrTexture* onGenerateTexture(GrContext* ctx, SkImageUsageType, const SkIRect* subset) override {
+    GrTexture* onGenerateTexture(GrContext* ctx, const GrTextureParams&,
+                                 const SkIRect* subset) override {
         // waiting on https://code.google.com/p/skia/issues/detail?id=4233
         return nullptr;
     }