DevTools: preserve computed properties expansion on Computed Style re-rendering

Today, when the computed styles widget gets updated, it collapses all the
previously expanded properties.

This patch preserves property expansion on computed style re-rendering.

BUG=623317
R=dgozman

Review-Url: https://codereview.chromium.org/2152313002
Cr-Commit-Position: refs/heads/master@{#405929}
5 files changed