DevTools: support TRACE_VALUE_TYPE_COPY_STRING

These strings need to be copied along with the event. We only used to
support static strings so far.

BUG=

Review URL: https://codereview.chromium.org/184303002

git-svn-id: svn://svn.chromium.org/blink/trunk@168363 bbb929c8-8fbe-4397-9dbb-9b2b20218538