DevTools: Do not cache row count on suggest box update.

This makes suggest box updating faster by postponing the calculation
of the row count that is only needed when page up/down is pressed.

R=pfeldman,apavlov,lushnikov
NOTRY=true

Review URL: https://codereview.chromium.org/250563002

git-svn-id: svn://svn.chromium.org/blink/trunk@172481 bbb929c8-8fbe-4397-9dbb-9b2b20218538