DevTools: update paint count on Layer details view

Update paint count for current selected layer on Layer details view,
after the layer did paint.

BUG=

Review URL: https://codereview.chromium.org/47963002

git-svn-id: svn://svn.chromium.org/blink/trunk@160699 bbb929c8-8fbe-4397-9dbb-9b2b20218538