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/tests/BlurTest.cpp b/tests/BlurTest.cpp
index f5bb215..556930e 100644
--- a/tests/BlurTest.cpp
+++ b/tests/BlurTest.cpp
@@ -170,8 +170,9 @@
 
     memset(src.fImage, 0xff, src.computeTotalImageSize());
 
-    dst.fImage = nullptr;
-    SkBlurMask::BlurGroundTruth(sigma, &dst, src, kNormal_SkBlurStyle);
+    if (!SkBlurMask::BlurGroundTruth(sigma, &dst, src, kNormal_SkBlurStyle)) {
+        return;
+    }
 
     int midX = dst.fBounds.centerX();
     int midY = dst.fBounds.centerY();