Add default ctor to SkMask

The minimal fix here seems to be handling BoxBlur's return value in SkBlurMaskFilter.cpp::GrRRectBlurEffect::Create. We seem to do enough special handling of the fImage field though that always initialializing it may not be a bad idea.

BUG=570232
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1539553002

Review URL: https://codereview.chromium.org/1539553002
diff --git a/gm/blurrect.cpp b/gm/blurrect.cpp
index 4f15815..55c592b 100644
--- a/gm/blurrect.cpp
+++ b/gm/blurrect.cpp
@@ -199,7 +199,10 @@
                     SkBlurStyle style = styles[k];
 
                     SkMask mask;
-                    SkBlurMask::BlurRect(SkBlurMask::ConvertRadiusToSigma(radius), &mask, r, style);
+                    if (!SkBlurMask::BlurRect(SkBlurMask::ConvertRadiusToSigma(radius),
+                                              &mask, r, style)) {
+                        continue;
+                    }
 
                     SkAutoMaskFreeImage amfi(mask.fImage);