DevTools: [SASS] poll CSS for 30 seconds instead of 5 seconds.

Currently, SASSSourceMapping polls CSS for 5 seconds after the SASS
change, and this turns out to be insufficient in case of complex
css-processing build steps.

Instead of adding one more setting "css polling duration", this patch
tries to solve most of the problems by increasing polling duration
to 30 seconds.

BUG=421925
R=vsevik, apavlov

Review URL: https://codereview.chromium.org/658123002

git-svn-id: svn://svn.chromium.org/blink/trunk@183819 bbb929c8-8fbe-4397-9dbb-9b2b20218538