mac: Fix a compile error when using libc++.

mac: Fix a compile error when using libc++.

Fixes a few errors that all look similar to:
third_party/llvm-build/Release+Asserts/include/c++/v1/map:1365:17: error:
    no matching function for call to object of type
    const content_setting_bubble::compare_button'
            if (__tree_.value_comp().key_comp()(__k, __nd->__value_.__cc.first))
                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
chrome/browser/ui/cocoa/content_settings/content_setting_bubble_cocoa.mm:573:16:
    note: in instantiation of member function 'std::__1::map<NSPopUpButton *,
          content_setting_bubble::MediaMenuParts *,
          content_setting_bubble::compare_button>>>::operator[]' requested here
    mediaMenus_[button] = menuParts;
               ^
chrome/browser/ui/cocoa/content_settings/content_setting_bubble_cocoa.h:43:8:
    note: candidate function not viable: 'this' argument has type
    'const content_setting_bubble::compare_button', but method is not marked
    const
  bool operator()(NSPopUpButton *const a, NSPopUpButton *const b) {
       ^
No behavior change.

BUG=400091
TBR=bauerb

Committed: https://crrev.com/7c7f4e37d368d46f16010a513acb22ccce63e61f
Cr-Commit-Position: refs/heads/master@{#348810}

[Nico, 2015-09-15 00:28:42]

thakis@chromium.org changed reviewers:
+ bauerb@chromium.org, jialiul@chromium.org

[Nico, 2015-09-15 00:28:42]

First to stamp wins!

[Jialiu Lin, 2015-09-15 00:55:02]

lgtm
On 2015/09/15 00:28:42, Nico wrote:
> First to stamp wins!

[Nico, 2015-09-15 00:57:13]

The CQ bit was checked by thakis@chromium.org

[commit-bot: I haz the power, 2015-09-15 00:58:44]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1341533007/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1341533007/1

[commit-bot: I haz the power, 2015-09-15 00:58:45]

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

[commit-bot: I haz the power, 2015-09-15 00:58:46]

No L-G-T-M from a valid reviewer yet. Only full committers are accepted.
Even if an L-G-T-M may have been provided, it was from a non-committer,
_not_ a full super star committer.
See http://www.chromium.org/getting-involved/become-a-committer
Note that this has nothing to do with OWNERS files.

[Nico, 2015-09-15 00:59:59]

The CQ bit was checked by thakis@chromium.org

[commit-bot: I haz the power, 2015-09-15 01:00:25]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1341533007/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1341533007/1

[commit-bot: I haz the power, 2015-09-15 01:35:23]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-15 01:35:57]

Patchset 1 (id:??) landed as
https://crrev.com/7c7f4e37d368d46f16010a513acb22ccce63e61f
Cr-Commit-Position: refs/heads/master@{#348810}

[Bernhard Bauer, 2015-09-15 08:12:35]

LGTM

On 2015/09/15 00:55:02, JialiuLin wrote:
> lgtm
> On 2015/09/15 00:28:42, Nico wrote:
> > First to stamp wins!

I humbly accept defeat. Truly, the better one has prevailed.

[commit-bot: I haz the power, 2015-09-23 12:41:07]

Patchset 1 (id:??) landed as
https://crrev.com/7c7f4e37d368d46f16010a513acb22ccce63e61f
Cr-Commit-Position: refs/heads/master@{#348810}