OnionSoup: Merge platform/wtf/wtf_thread_data to platform/wtf/threading

As a step to acheive OnionSoup's goal, this CL merges WTFThreadData into
Threading. Major changes are as below,

 - Rename WTFThreadData class to Threading class.
 - Rename WtfThreadData() function to WtfThreading() function.
 - Remove wtf_thread_data.h/cc files.
 - Remove empty namespace in std_lib_extras.h

Bug: 950131
Change-Id: Ib120a3dc61fcb4559740db85cf4075e828bec1f1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1558836
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Gyuyoung Kim <gyuyoung@igalia.com>
Cr-Commit-Position: refs/heads/master@{#649020}
15 files changed