Calling Keychecker before checking key in MessageMap

[cyyber]

This is fix regarding the issue #4029

[grpc-kokoro]

Thanks for your pull request. The automated tests will run as soon as one of the admins verifies this change is ok for us to run on our infrastructure.

[grpc-kokoro]

Thanks for your pull request. The automated tests will run as soon as one of the admins verifies this change is ok for us to run on our infrastructure.

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[cyyber]

I signed it!

[googlebot]

CLAs look good, thanks!

[acozzette]

ok to test (this comment will start the other CI runs)

[anandolee]

Please remove the TODO as well

[cyyber]

Ok

[cyyber]

TODO has been removed. @anandolee

[anandolee]

The change is OK to merge after 'remove the TODO' and jenkins test.