We often rgn-diff an area >= the other rgn. now we detect that and return empty

We often rgn-diff an area >= the other rgn. now we detect that and return empty
We do this when we update our devices in SkCanvas.cpp
Committed: https://code.google.com/p/skia/source/detail?r=4101

[TomH, 2012-05-31 18:23:15]

LGTM

https://codereview.appspot.com/6249073/diff/2001/bench/RegionBench.cpp
File bench/RegionBench.cpp (right):

https://codereview.appspot.com/6249073/diff/2001/bench/RegionBench.cpp#newcode35
bench/RegionBench.cpp:35: return result.op(a, a.getBounds(),
SkRegion::kDifference_Op);
Not even referring to b looks really odd - is it worth a comment?