Minor tweaks to ByteSwap.h

- Remove duplicate comment.
- Correct comment about GCC.
- Use ALWAYS_INLINE for the function definitions backed by intrinsics.

Review URL: https://codereview.chromium.org/106963003

git-svn-id: svn://svn.chromium.org/blink/trunk@163493 bbb929c8-8fbe-4397-9dbb-9b2b20218538