Reland: Move wtf_unittests to platform/wtf/.

This relands the exact same patch as the previous commit
https://codereview.chromium.org/2771783003 did. This patch was reverted
because the offending CL was dependent on it.

TBRing since the exact same change was approved before.

> This patch moves all the cpp files for wtf_unittests to their new home
> platform/wtf. The target "wtf_unittests" is removed from wtf/BUILD.gn, and
> it's added in platform/wtf/BUILD.gn. Due to this change, a few other files
> need to be updated, too.

BUG=691465
TBR=haraken@chromium.org,thakis@chromium.org

Review-Url: https://codereview.chromium.org/2787293002
Cr-Original-Commit-Position: refs/heads/master@{#461072}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 6fbccafc5d50e35d8f8360559829a8b5ece100dc
38 files changed