base::CircularQueue give up on const correctness for MSVC

This CL makes the CircularQueue iterator not work on const
queue instances (for which there is no current usage) in exchange
of making it work on MSVC.
The problem is that the current Iterator implementation is only
half const-correct. A truly const-correct version would require
two Iterator classes a const_iterator and an interator, and
corresponding const+non-const begin()/end() and front()/back()
methods on the CircularQueue container. That's too much complexity
and more chances to get things wrong.

Failure log: https://pastebin.com/raw/2v3q6pkf
FAILED: obj/src/base/unittests.circular_queue_unittest.obj
C:\Program Files (x86)\Microsoft Visual Studio\2019\BuildTools\VC\Tools\MSVC\14.28.29333\bin\Hostx64\x64\cl.exe /nologo /showIncludes /FC -D_CRT_NONSTDC_NO_WARNINGS -D_CRT_SECURE_NO_DEPRECATE -D_CRT_SECURE_NO_WARNINGS -D_SCL_SECURE_NO_DEPRECATE -DNOMINMAX -D_HAS_EXCEPTIONS=0 -DWIN32_LEAN_AND_MEAN -DGTEST_LANG_CXX11=1 -DPERFETTO_IMPLEMENTATION "-IC:/Program Files (x86)/Microsoft Visual Studio/2019/BuildTools/VC/Tools/MSVC/14.28.29333/include" "-IC:/Program Files (x86)/Windows Kits/10/Include/10.0.19041.0/ucrt" "-IC:/Program Files (x86)/Windows Kits/10/Include/10.0.19041.0/um" "-IC:/Program Files (x86)/Windows Kits/10/Include/10.0.19041.0/shared" -I../../buildtools/googletest/googletest/include -I../../buildtools/googletest/googlemock/include -I../.. -I../../include -Igen/build_config -Igen -I../../src/third_party_getopt /Od /bigobj /Gy /WX /Z7 /W2 /wd4244 /wd4267 /GR- /c ../../src/base/circular_queue_unittest.cc /Foobj/src/base/unittests.circular_queue_unittest.obj /Fd"obj/src/base/unittests_c.pdb"
C:\Program Files (x86)\Microsoft Visual Studio\2019\BuildTools\VC\Tools\MSVC\14.28.29333\include\algorithm(7419): error C3892: '_First': you cannot assign to a variable that is const
C:\Program Files (x86)\Microsoft Visual Studio\2019\BuildTools\VC\Tools\MSVC\14.28.29333\include\algorithm(7541): note: see reference to function template instantiation '_BidIt std::_Insertion_sort_unchecked<_RanIt,_Pr>(const _BidIt,const _BidIt,_Pr)' being compiled
        with
        [
            _BidIt=perfetto::base::CircularQueue<uint64_t>::Iterator,
            _RanIt=perfetto::base::CircularQueue<uint64_t>::Iterator,
            _Pr=std::less<void>
        ]
C:\Program Files (x86)\Microsoft Visual Studio\2019\BuildTools\VC\Tools\MSVC\14.28.29333\include\algorithm(7571): note: see reference to function template instantiation 'void std::_Sort_unchecked<_RanIt,_Fn>(_RanIt,_RanIt,__int64,_Pr)' being compiled
        with
        [
            _RanIt=perfetto::base::CircularQueue<uint64_t>::Iterator,
            _Fn=std::less<void>,
            _Pr=std::less<void>
        ]
C:\Program Files (x86)\Microsoft Visual Studio\2019\BuildTools\VC\Tools\MSVC\14.28.29333\include\algorithm(7576): note: see reference to function template instantiation 'void std::sort<_RanIt,std::less<void>>(const _RanIt,const _RanIt,_Pr)' being compiled
        with
        [
            _RanIt=perfetto::base::CircularQueue<uint64_t>::Iterator,
            _Pr=std::less<void>
        ]
D:\Progetti\perfetto\src\base\circular_queue_unittest.cc(100): note: see reference to function template instantiation 'void std::sort<perfetto::base::CircularQueue<uint64_t>::Iterator>(const _RanIt,const _RanIt)' being compiled
        with
        [
            _RanIt=perfetto::base::CircularQueue<uint64_t>::Iterator
        ]
ninja: build stopped: subcommand failed.
PS D:\Progetti\perfetto>

Bug: 174454879
Change-Id: I1cfb44dac646f455157ce08b15e14b3349e5a7de