Use if instead of ifdef SK_SUPPORT_GPU

The SK_SUPPORT_GPU is always defined.

R=reed@google.com, bsalomon@google.com

Author: kkinnunen@nvidia.com

Review URL: https://codereview.chromium.org/331353003
diff --git a/include/core/SkImageFilter.h b/include/core/SkImageFilter.h
index cd34ba3..e7ba5e6 100644
--- a/include/core/SkImageFilter.h
+++ b/include/core/SkImageFilter.h
@@ -172,7 +172,7 @@
     // Default impl returns union of all input bounds.
     virtual void computeFastBounds(const SkRect&, SkRect*) const;
 
-#ifdef SK_SUPPORT_GPU
+#if SK_SUPPORT_GPU
     /**
      * Wrap the given texture in a texture-backed SkBitmap.
      */