blink: Enforce ParkableString thread restrictions when DCHECK_IS_ON().

Bug: 877044
Change-Id: I6b3e7be16ade21bd9c511a6a4fda10930575efa4
Reviewed-on: https://chromium-review.googlesource.com/1210782
Commit-Queue: Benoit L <lizeb@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589505}
2 files changed