use (temp) gpu-imagefilter-cache for applyFilter

BUG=skia:

Review URL: https://codereview.chromium.org/1414653003
diff --git a/src/gpu/SkGpuDevice.h b/src/gpu/SkGpuDevice.h
index 2e8d14e..b2031e8 100644
--- a/src/gpu/SkGpuDevice.h
+++ b/src/gpu/SkGpuDevice.h
@@ -135,6 +135,8 @@
                        const SkImageFilter::Context&,
                        SkBitmap* result, SkIPoint* offset);
 
+    static SkImageFilter::Cache* NewImageFilterCache();
+
 protected:
     bool onReadPixels(const SkImageInfo&, void*, size_t, int, int) override;
     bool onWritePixels(const SkImageInfo&, const void*, size_t, int, int) override;