Clean up shadow assert.

Clean up shadow assert.

When drawing an analytic rrect shadow, the mask filter could pass
the devRect into the drawOp. If we have a small scale that could
make the radii less than SK_ScalarNearlyZero. We might also have
a rect that we outset by a value less than SK_ScalarNearlyZero to
the penumbra border. So the best we can check is that the the two
radii are equal.

Bug: skia:6608
Change-Id: I8bdafe089b302fbc5c9042ff30798d3688e9e781
Reviewed-on: https://skia-review.googlesource.com/16364
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>