Fast path for ThreadSpecific for main thread on TLS-slow platforms

This code is mainly a refactor of utility code out of platform/heap and
into wtf/StackUtil. ThreadSpecific then copies some tricks done by
ThreadState.

BUG=621786
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2623273007
Cr-Commit-Position: refs/heads/master@{#444581}
12 files changed