DevTools: Do not force style sheets update on inspector start.

This also fixes the case case when inspector was closed and then stylesheet was removed on the inspected page
In this case inspector frontend received an incorrect styleSheetRemoved event after reopening.

Review URL: https://codereview.chromium.org/121263002

git-svn-id: svn://svn.chromium.org/blink/trunk@164379 bbb929c8-8fbe-4397-9dbb-9b2b20218538