IntToString() signedness fixes for //dbus

IntToString() signedness fixes for //dbus

Use UintToString() rather than IntToString() for unsigned types, to
improve correctness.

Also replace some trivial usages of StringPrintf() with the appropriate
IntToString() variants, to improve performance, code size and type
safety.

No behavioural changes are intended.

BUG=505479
TEST=dbus_unittests

Committed: https://crrev.com/19b947df2525b66bee8fae31de23dba12423703a
Cr-Commit-Position: refs/heads/master@{#349785}

[Adam Rice, 2015-09-18 21:18:54]

ricea@chromium.org changed reviewers:
+ stevenjb@chromium.org

[stevenjb, 2015-09-18 21:26:09]

lgtm

[Adam Rice, 2015-09-18 21:55:59]

The CQ bit was checked by ricea@chromium.org

[commit-bot: I haz the power, 2015-09-18 21:56:59]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1349493004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1349493004/1

[commit-bot: I haz the power, 2015-09-18 22:26:17]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-18 22:27:00]

Patchset 1 (id:??) landed as
https://crrev.com/19b947df2525b66bee8fae31de23dba12423703a
Cr-Commit-Position: refs/heads/master@{#349785}