Adjust the default timeout for debug builds to reflect reality.

Currently content_shell is 10x slower in debug mode than release.
The previous code only adjusted the timeout to be 2x slower, and that
led us to mark a lot of tests as either slow or timeout unfairly.

R=abarth@chromium.org, ojan@chromium.org, esprehn@chromium.org
BUG=254188

Review URL: https://chromiumcodereview.appspot.com/18566004

git-svn-id: svn://svn.chromium.org/blink/trunk@155277 bbb929c8-8fbe-4397-9dbb-9b2b20218538