Fix perf regression in blink_perf.shadow_dom.

Fix perf regression in blink_perf.shadow_dom.

The static strings were destructed at the exit instead of leaked,
causing performance regression. Since the GCInfo objects are leaked,
holding a reference in the struct will leak the strings.

BUG=520948
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=200944

[ssid, 2015-08-20 10:07:30]

ssid@chromium.org changed reviewers:
+ haraken@chromium.org

[ssid, 2015-08-20 10:07:31]

PTAL.
Sorry for the roundtrips, I think this will fix all the issues. Please tell me
if you have better suggestions, thanks.

[haraken, 2015-08-20 11:03:47]

LGTM, thanks for working on this!

[ssid, 2015-08-20 11:22:29]

The CQ bit was checked by ssid@chromium.org

[commit-bot: I haz the power, 2015-08-20 11:22:40]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1300263003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1300263003/1

[commit-bot: I haz the power, 2015-08-20 12:46:15]

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

[commit-bot: I haz the power, 2015-08-20 12:46:16]

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

[ssid, 2015-08-21 00:22:46]

The CQ bit was checked by ssid@chromium.org

[commit-bot: I haz the power, 2015-08-21 00:23:29]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1300263003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1300263003/1

[commit-bot: I haz the power, 2015-08-21 00:31:48]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=200944