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

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

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

BUG=367418
TEST=
R=jam@chromium.org,jochen@chromium.org

Committed: https://crrev.com/9f9f5da774a8449eb00fe49e4b97916f3d3ed07d
Cr-Commit-Position: refs/heads/master@{#350044}

[jochen (gone - plz use gerrit), 2015-09-21 15:03:10]

lgtm

[ki.stfu, 2015-09-21 17:15:34]

The CQ bit was checked by ki.stfu@gmail.com

[commit-bot: I haz the power, 2015-09-21 17:16:14]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1345393005/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1345393005/1

[commit-bot: I haz the power, 2015-09-21 17:42:11]

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

[commit-bot: I haz the power, 2015-09-21 17:42:11]

Try jobs failed on following builders:
  win8_chromium_ng on tryserver.chromium.win (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.win/builders/win8_chromium_ng/...)

[ki.stfu, 2015-09-21 21:13:33]

The CQ bit was checked by ki.stfu@gmail.com to run a CQ dry run

[commit-bot: I haz the power, 2015-09-21 21:14:47]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1345393005/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1345393005/1

[commit-bot: I haz the power, 2015-09-21 21:45:24]

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

[commit-bot: I haz the power, 2015-09-21 21:45:25]

Dry run: This issue passed the CQ dry run.

[ki.stfu, 2015-09-21 22:11:46]

The CQ bit was checked by ki.stfu@gmail.com

[commit-bot: I haz the power, 2015-09-21 22:12:22]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1345393005/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1345393005/1

[commit-bot: I haz the power, 2015-09-21 22:20:10]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-21 22:21:57]

Patchset 1 (id:??) landed as
https://crrev.com/9f9f5da774a8449eb00fe49e4b97916f3d3ed07d
Cr-Commit-Position: refs/heads/master@{#350044}