hide virtuals on device for width/height/isopaque

R=bsalomon@google.com
NOTREECHECKS=True

Author: reed@chromium.org

Review URL: https://codereview.chromium.org/334993002
diff --git a/include/core/SkImageInfo.h b/include/core/SkImageInfo.h
index d66158d..b955433 100644
--- a/include/core/SkImageInfo.h
+++ b/include/core/SkImageInfo.h
@@ -176,6 +176,13 @@
         return info;
     }
 
+    static SkImageInfo MakeUnknown() {
+        SkImageInfo info = {
+            0, 0, kUnknown_SkColorType, kIgnore_SkAlphaType
+        };
+        return info;
+    }
+
     int width() const { return fWidth; }
     int height() const { return fHeight; }
     SkColorType colorType() const { return fColorType; }