Cleanup: Pass std::string as const reference from ipc/

Cleanup: Pass std::string as const reference from ipc/

Passing std::string by reference can prevent extra copying of object.

BUG=367418
TEST=
R=agl@chromium.org,darin@chromium.org,cpu@chromium.org,jeremy@chromium.org

Committed: https://crrev.com/5e7a0f7968d389c782d56b87846c3cd34238193e
Cr-Commit-Position: refs/heads/master@{#350942}

[ki.stfu, 2015-09-25 11:26:06]

ki.stfu@gmail.com changed reviewers:
+ cpu@chromium.org, jeremy@chromium.org

[ki.stfu, 2015-09-25 11:26:25]

ping

[cpu_(ooo_6.6-7.5), 2015-09-25 22:39:00]

lgtm

[agl, 2015-09-25 22:43:29]

lgtm

[agl, 2015-09-25 22:46:40]

The CQ bit was checked by agl@chromium.org

[agl, 2015-09-25 22:46:41]

lgtm

[commit-bot: I haz the power, 2015-09-25 22:47:30]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1350163006/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1350163006/1

[commit-bot: I haz the power, 2015-09-25 23:17:30]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-25 23:18:20]

Patchset 1 (id:??) landed as
https://crrev.com/5e7a0f7968d389c782d56b87846c3cd34238193e
Cr-Commit-Position: refs/heads/master@{#350942}