Reduce dynamicscompressor threshold.

This allows the test to pass on Windows, but still detects that
pre-emphasis is off.

BUG=349523

Review URL: https://codereview.chromium.org/314743002

git-svn-id: svn://svn.chromium.org/blink/trunk@175536 bbb929c8-8fbe-4397-9dbb-9b2b20218538