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
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Review URL: https://codereview.chromium.org/1539553002
15 files changed