Remove external SkImageFilter cache, and rename UniqueIDCache -> Cache.

There Can Only Be One.... Cache for SkImageFilter.

R=bsalomon@google.com

BUG=skia:

Author: senorblanco@chromium.org

Review URL: https://codereview.chromium.org/452923002
diff --git a/include/gpu/SkGpuDevice.h b/include/gpu/SkGpuDevice.h
index b43213a..5ff26f6 100644
--- a/include/gpu/SkGpuDevice.h
+++ b/include/gpu/SkGpuDevice.h
@@ -169,7 +169,7 @@
 
     virtual SkSurface* newSurface(const SkImageInfo&) SK_OVERRIDE;
 
-    virtual SkImageFilter::UniqueIDCache* getImageFilterCache() SK_OVERRIDE;
+    virtual SkImageFilter::Cache* getImageFilterCache() SK_OVERRIDE;
 
     // sets the render target, clip, and matrix on GrContext. Use forceIdenity to override
     // SkDraw's matrix and draw in device coords.