Remove non-thread-safe assertions and comments for AtomicString

WTF Strings and AtomicString are now thread-safe. This CL removes some
comment warnings about their thread-safety and removes the owner thread
DCHECKS in string_resource.cc.

Note that ParkableStrings are not  thread-safe and their illegal usage
from StringResource will still trigger assertions in
ParkableString::AssertOnValidThread.

Bug: 1083392
Change-Id: I72763d9a2efe1c19c2a19afd57e2ba782d8ceb10
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3887223


Commit-Queue: Luis Fernando Pardo Sixtos <lpardosixtos@microsoft.com>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1045911}