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According to comment in base/port.h, in Chromium, we force-define
__STDC_CONSTANT_MACROS, so we can just use the regular (U)INTn_C macros from
<stdint.h>.

BUG=138542
TEST=base_unittests
R=danakj@chromium.org

Review URL: https://codereview.chromium.org/1128203010

Cr-Commit-Position: refs/heads/master@{#330440}