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

Review-Url: https://codereview.chromium.org/2771783003
Cr-Commit-Position: refs/heads/master@{#459715}