Fix illegal use of const in container contents to unbreak VS 2015.

Fix illegal use of const in container contents to unbreak VS 2015.

For details see the STL Breaking Changes (VS14 CTP1) section of:
http://blogs.msdn.com/b/vcblog/archive/2014/06/06/
c-14-stl-features-fixes-and-breaking-changes-in-
visual-studio-14-ctp1.aspx

In particular:
"The Standard has always forbidden containers of const elements...
...elements must be Assignable, which const T isn't."

Error message is:
error C2338: The C++ Standard forbids containers of const
elements because allocator<const T> is ill-formed.

R=cpu@chromium.org
BUG=440500
Committed: https://crrev.com/51428030f98124b4ec5d7c16d9b99e13970c2621
Cr-Commit-Position: refs/heads/master@{#324339}

[cpu_(ooo_6.6-7.5), 2015-04-09 00:01:46]

lgtm

[cpu_(ooo_6.6-7.5), 2015-04-09 00:01:48]

The CQ bit was checked by cpu@chromium.org

[commit-bot: I haz the power, 2015-04-09 00:03:31]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1066213002/1

[commit-bot: I haz the power, 2015-04-09 02:12:53]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-09 02:13:43]

Patchset 1 (id:??) landed as
https://crrev.com/51428030f98124b4ec5d7c16d9b99e13970c2621
Cr-Commit-Position: refs/heads/master@{#324339}