Turn MQ classes into thread safe

Turns the MQ evaluation classes (MediaQuery, MediaQueryExp and MediaQueryEvaluator) to thread safe by switching them from using AtomicStrings to static strings, initialized by Init.

This CL is a split of https://codereview.chromium.org/171383002/

BUG=

Review URL: https://codereview.chromium.org/178803006

git-svn-id: svn://svn.chromium.org/blink/trunk@168682 bbb929c8-8fbe-4397-9dbb-9b2b20218538