Handle Performance object creation on loader-less Documents.

Handle Performance object creation on loader-less Documents.

Speculatively fix reported null crasher in Performance ctor,
checking if Document has a DocumentLoader before accessing.
Should this object be created while its frame is being detached
(see bug stack trace), this can potentially happen.

R=haraken,ksakamoto
BUG=518775
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=200307

[sof, 2015-08-11 06:12:07]

sigbjornf@opera.com changed reviewers:
+ haraken@chromium.org, ksakamoto@chromium.org

[sof, 2015-08-11 06:12:07]

please take a look.

I don't think this is overly defensive, but I haven't been able to repro either.
Including ksakamoto@ due to recent Performance object improvements & knowledge
(I don't think those changes are potentially a cause here, though.)

[haraken, 2015-08-11 06:25:02]

LGTM

[Kunihiko Sakamoto, 2015-08-11 06:27:22]

lgtm

[sof, 2015-08-11 08:15:12]

The CQ bit was checked by sigbjornf@opera.com

[commit-bot: I haz the power, 2015-08-11 08:15:16]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1287513003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1287513003/1

[commit-bot: I haz the power, 2015-08-11 08:17:55]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=200307