Implement CheckedNumeric::UnsignedAbs()

Implement CheckedNumeric::UnsignedAbs()

This covers some corner cases where we want an equivalent to
std::abs() for signed integers, but we want it to convert to
an unsigned type of the same width and properly handle the
value of signed min().

Committed: https://crrev.com/eaf375f8db272522afff267fe92fa6b27012a78f
Cr-Commit-Position: refs/heads/master@{#349300}

[Jeffrey Yasskin, 2015-09-15 23:58:25]

jyasskin@chromium.org changed reviewers:
+ jyasskin@chromium.org

[Jeffrey Yasskin, 2015-09-15 23:58:25]

lgtm, or at least, I don't see the problems.

https://codereview.chromium.org/1346753002/diff/20001/base/numerics/safe_math...
File base/numerics/safe_math_impl.h (right):

https://codereview.chromium.org/1346753002/diff/20001/base/numerics/safe_math...
base/numerics/safe_math_impl.h:327: // Absolute value of a positive is just its
identiy.
"T is unsigned, so |value| must already be positive."?

[jschuh, 2015-09-16 16:00:21]

The CQ bit was checked by jschuh@chromium.org

[jschuh, 2015-09-16 16:00:21]

The patchset sent to the CQ was uploaded after l-g-t-m from
jyasskin@chromium.org
Link to the patchset: https://codereview.chromium.org/1346753002/#ps40001
(title: "comments")

[commit-bot: I haz the power, 2015-09-16 16:01:13]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1346753002/40001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1346753002/40001

[commit-bot: I haz the power, 2015-09-16 16:16:34]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-09-16 16:16:34]

Try jobs failed on following builders:
  cast_shell_android on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/cast_shell_andr...)

[jschuh, 2015-09-16 19:40:17]

The CQ bit was checked by jschuh@chromium.org

[jschuh, 2015-09-16 19:40:17]

The patchset sent to the CQ was uploaded after l-g-t-m from
jyasskin@chromium.org
Link to the patchset: https://codereview.chromium.org/1346753002/#ps80001
(title: "gcc/glang build fix")

[jschuh, 2015-09-16 19:40:35]

Patchset #4 (id:60001) has been deleted

[commit-bot: I haz the power, 2015-09-16 19:41:54]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1346753002/80001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1346753002/80001

[commit-bot: I haz the power, 2015-09-16 20:06:03]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-09-16 20:06:04]

Try jobs failed on following builders:
  mac_chromium_compile_dbg_ng on tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/mac_chromium_comp...)
  mac_chromium_rel_ng on tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/mac_chromium_rel_...)

[jschuh, 2015-09-16 22:13:55]

The CQ bit was checked by jschuh@chromium.org

[jschuh, 2015-09-16 22:13:55]

The patchset sent to the CQ was uploaded after l-g-t-m from
jyasskin@chromium.org
Link to the patchset: https://codereview.chromium.org/1346753002/#ps100001
(title: "gcc build fix")

[commit-bot: I haz the power, 2015-09-16 22:14:23]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1346753002/100001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1346753002/100001

[commit-bot: I haz the power, 2015-09-17 01:04:34]

Committed patchset #5 (id:100001)

[commit-bot: I haz the power, 2015-09-17 01:05:12]

Patchset 5 (id:??) landed as
https://crrev.com/eaf375f8db272522afff267fe92fa6b27012a78f
Cr-Commit-Position: refs/heads/master@{#349300}