[asm.js] Cleanup namespace usage in AsmJs class.

R=clemensh@chromium.org

Change-Id: I3aad1b4058eb464dee80c08332a186109efb7208
Reviewed-on: https://chromium-review.googlesource.com/485622
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44829}