Platformize UnsafeArena in SPDY code

Merge internal change: 228810977

R=rch@chromium.org

Change-Id: Iaa05eb7fe97ea0c3d326c8530d52be8a2a7573cd
Reviewed-on: https://chromium-review.googlesource.com/c/1406162
Reviewed-by: Ryan Hamilton <rch@chromium.org>
Commit-Queue: Victor Vasiliev <vasilvv@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622140}
7 files changed