Fix compilation error with older versions of libstdc++
Fix compilation error with older versions of libstdc++

Some Chromecast builders were failing, since the test for __GLIBCXX__
was causing extra comparison operator overloads to be included for
nullptr_t, but our version of libstdc++ already defines those overrides.

BUG=

Review URL: https://codereview.chromium.org/1771403004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#380034}

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