ui/gfx: Make gfx::Size::GetArea use checked math.

ui/gfx: Make gfx::Size::GetArea use checked math.

This patch ensures that if the area calculation would overflow, we
instead fail a CHECK.

R=danakj, pkasting@chromium.org
BUG=352761
Committed: https://crrev.com/35f226abcae1a7143e9574b31e391a8e8e0c26ae
Cr-Commit-Position: refs/heads/master@{#353594}

[vmpstr, 2015-09-25 22:34:13]

Please take a look.

[vmpstr, 2015-09-25 22:38:48]

On 2015/09/25 22:34:13, vmpstr wrote:
> Please take a look.

ash/display has a few instances of sorting based on area. This means that every
comparison would do this checked math. I don't know if that's going to cause any
slowdowns.

[Peter Kasting, 2015-09-25 23:03:02]

LGTM.  This checked math should be very cheap, so I'm not worried about perf
impact.

As noted on the bug, it may still make sense to have some kind of
"GetSizeInBytes()" type accessor to ensure that people still don't overflow when
multiplying this area by a bytes-per-pixel value, but this seems fine
regardless.

[vmpstr, 2015-10-02 18:11:59]

vmpstr@chromium.org changed reviewers:
+ vollick@chromium.org

[vmpstr, 2015-10-02 18:11:59]

danakj@, vollick@, can you take a look please?

[danakj, 2015-10-06 20:27:57]

LGTM

[vmpstr, 2015-10-12 20:59:10]

The CQ bit was checked by vmpstr@chromium.org

[commit-bot: I haz the power, 2015-10-12 21:00:23]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1371753004/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1371753004/20001

[commit-bot: I haz the power, 2015-10-12 21:08:29]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-10-12 21:08:55]

Patchset 2 (id:??) landed as
https://crrev.com/35f226abcae1a7143e9574b31e391a8e8e0c26ae
Cr-Commit-Position: refs/heads/master@{#353594}