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

Review URL: https://codereview.chromium.org/1341533007

Cr-Commit-Position: refs/heads/master@{#348810}