Remove emphasis/de-emphasis filters from DynamicsCompressor.

The pre-emphasis and de-emphasis filters are specified in the WebAudio spec, and this causes problems for Chrome OS which is using WebAudio to test speakers.

A discussion on the W3C audio list indicates a consensus to remove the pre-emphasis and de-emphasis filters since they were never specified anyway:

http://lists.w3.org/Archives/Public/public-audio/2014JanMar/0010.html

A simple layout test is added.  Before this CL, the output is compressed; with this CL, the output is no longer compressed.

See also http://webaudiodemos.appspot.com/compressorfreq/ which demonstrates the compression that is happening.  With this CL, the sine wave remains at a constant amplitude.  Previously, the sine wave amplitude decreases as the frequency increases.

BUG=340870

Review URL: https://codereview.chromium.org/152333003

git-svn-id: svn://svn.chromium.org/blink/trunk@166956 bbb929c8-8fbe-4397-9dbb-9b2b20218538