string_number_conversions: only check CXX_LIBRARY_HAS_CONSTEXPR.
string_number_conversions: only check CXX_LIBRARY_HAS_CONSTEXPR.

CXX_LIBRARY_VERSION is irrelevant, because the only C++11 library
feature of any concern is whether numeric_limits’ min() and max() are
declared constexpr.

Crashpad is C++11-only as far as the language is concerned, and the
comment doesn’t need to call it out explicitly because static_assert()
is always available.

R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/809833002

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