base: Use compiler builtins for byte swapping

Blink/WTF already makes use of compiler builtins for endian byte
swapping so do the same in Chromium base itself.

BUG=620779
TEST=base_unittests:ByteOrderTest.*

Review-Url: https://codereview.chromium.org/2077563002
Cr-Commit-Position: refs/heads/master@{#400540}
4 files changed