Watch out for SkFixed overflow in SkMipMap.cpp.

Watch out for SkFixed overflow in SkMipMap.cpp.

Tested with -fsanitize=signed-integer-overflow.
This new assert used to trigger in MipMap unit test.

Don't appear to be any GM diffs.

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/09a22e9597e271c44dc7c2b71c72cf62a7de1e19

[mtklein, 2014-11-17 17:56:01]

mtklein@google.com changed reviewers:
+ mtklein@google.com, reed@google.com

[reed1, 2014-11-21 17:10:48]

lgtm w/ nit for comment or diff symbol

https://codereview.chromium.org/729373004/diff/1/include/core/SkTypes.h
File include/core/SkTypes.h (right):

https://codereview.chromium.org/729373004/diff/1/include/core/SkTypes.h#newco...
include/core/SkTypes.h:355: SkASSERT(value != -2147483648);
nit: either 0x80000000 or SK_NaN32 might be clearer (plus maybe a comment why
this is tricky for abs.

[mtklein, 2014-11-21 19:26:56]

Done.

[mtklein, 2014-11-21 19:27:01]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2014-11-21 19:27:30]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/729373004/20001

[commit-bot: I haz the power, 2014-11-21 19:38:56]

Committed patchset #2 (id:20001) as
https://skia.googlesource.com/skia/+/09a22e9597e271c44dc7c2b71c72cf62a7de1e19