Introduce WTF::releaseStore and WTF::acquireLoad.

Use them to avoid data races in the SafePointBarrier discovered by TSAN and guarantee that compiler does not move any memory accesses around m_unparkedThreadCount and m_canResume loads.

This change is likely to affect only ARM with its weakly ordered memory model.

BUG=347559

Review URL: https://codereview.chromium.org/199413012

git-svn-id: svn://svn.chromium.org/blink/trunk@169469 bbb929c8-8fbe-4397-9dbb-9b2b20218538