Fix possible integer overflow in SkTSearch's midpoint calculation

Fix possible integer overflow in SkTSearch's midpoint calculation

Probably unlikely to matter in practice, but SkTQSort calculates its
midpoint correctly, so we might as well do it here too.  For more
background, see Joshua Bloch's "Nearly All Binary Searches and
Mergesorts are Broken" post:
http://googleresearch.blogspot.ru/2006/06/extra-extra-read-all-about-it-nearly.html

This doesn't change any public API.
TBR=reed@google.com

Committed: https://skia.googlesource.com/skia/+/07ed41fa405fe5f22c85f2fc9935397bbaa9c13b

[mdempsky, 2015-09-22 15:34:15]

The CQ bit was checked by mdempsky@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-09-22 15:34:26]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1362613002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1362613002/1

[commit-bot: I haz the power, 2015-09-22 15:34:27]

Note for Reviewers:
The CQ is waiting for an approval.
If you believe that the CL is not ready yet, or if you would like to L-G-T-M
with comments then please uncheck the CQ checkbox.

Waiting for LGTM from valid reviewer(s) till 2015-09-22 21:34 UTC

[commit-bot: I haz the power, 2015-09-22 15:35:15]

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

[commit-bot: I haz the power, 2015-09-22 15:35:16]

Dry run: Try jobs failed on following builders:
  Build-Ubuntu-GCC-Arm64-Debug-Android-Trybot on client.skia.compile
(JOB_FAILED,
http://build.chromium.org/p/client.skia.compile/builders/Build-Ubuntu-GCC-Arm...)
  Build-Ubuntu-GCC-x86_64-Release-Trybot on client.skia.compile (JOB_FAILED,
http://build.chromium.org/p/client.skia.compile/builders/Build-Ubuntu-GCC-x86...)

[mdempsky, 2015-09-22 15:38:08]

The CQ bit was checked by mdempsky@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-09-22 15:38:24]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1362613002/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1362613002/20001

[mtklein, 2015-09-22 16:05:26]

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

[mtklein, 2015-09-22 16:05:26]

lgtm

This doesn't change any public API, so feel free to add:

"
This doesn't change any public API.
TBR=reed@google.com
"

to get the presubmit about public API changes to shut up.

[commit-bot: I haz the power, 2015-09-22 16:05:51]

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

[commit-bot: I haz the power, 2015-09-22 16:05:51]

Dry run: This issue passed the CQ dry run.

[mdempsky, 2015-09-22 16:13:31]

On 2015/09/22 16:05:26, mtklein wrote:
> lgtm
> 
> This doesn't change any public API, so feel free to add:
> 
> "
> This doesn't change any public API.
> mailto:TBR=reed@google.com
> "
> 
> to get the presubmit about public API changes to shut up.

Hm, done, though I don't recall seeing any presubmit warnings to that effect. 
Where would they have been?

[mtklein, 2015-09-22 17:31:06]

On 2015/09/22 16:13:31, mdempsky wrote:
> On 2015/09/22 16:05:26, mtklein wrote:
> > lgtm
> > 
> > This doesn't change any public API, so feel free to add:
> > 
> > "
> > This doesn't change any public API.
> > mailto:TBR=reed@google.com
> > "
> > 
> > to get the presubmit about public API changes to shut up.
> 
> Hm, done, though I don't recall seeing any presubmit warnings to that effect. 
> Where would they have been?

They'd happen right now as I click the commit box.  The CQ dry run intentionally
does not run the presubmit.

[mtklein, 2015-09-22 17:31:13]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2015-09-22 17:31:24]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1362613002/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1362613002/20001

[commit-bot: I haz the power, 2015-09-22 17:32:05]

Committed patchset #2 (id:20001) as
https://skia.googlesource.com/skia/+/07ed41fa405fe5f22c85f2fc9935397bbaa9c13b