This patch also changes String body descriptor to use synchronized
length and adds atomic live_bytes accessor.

BUG=chromium:694255

Change-Id: I41233b2097ec5c6a4ea2c45d4b8febf7ffca155e
Reviewed-on: https://chromium-review.googlesource.com/527093Reviewed-by: Hannes Payer <hpayer@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45761}