DevTools: [SSP] fix race condition during property editing.

StylePropertyTreeElement.applyStyleText method initiates a request to
the backend to set property text. If the round-trip takes significant
amount of time, a consequent call to applyStyleText function will result
in undefined behavior.

This patch serializes calls to the applyStyleText method via throttler.

BUG=413538
R=vsevik, apavlov@chromium.org

Review URL: https://codereview.chromium.org/565323003

git-svn-id: svn://svn.chromium.org/blink/trunk@182441 bbb929c8-8fbe-4397-9dbb-9b2b20218538