The issue is summarized here:  https://github.com/WebAudio/web-audio-api/issues/324

This CL can't solve all issues when the source position approaches the listener position because when the locations are equal, we get a singular point in the equations.

This just makes it continuous as the source approaches the listener. If the source then goes past, there will be a unavoidable discontinuity.

TEST=Listen to http://jsfiddle.net/UxwGP/10. When the slider position is 0, there should be no sudden change in pitch.

TEST=Listen to http://jsfiddle.net/qWHAt/34/ using mono source and HRTF panner.  Initially, the pitch is very low. Move the slider just to the left or right of center and the pitch goes up. As you move the slider through center slowly, the pitch drops drastically at center.  With this CL, this the pitch does not start out low, and as you move the slider around, the pitch never drops low at center. There is a discontinuity moving from left to right, but this is because the source was moving away from the listener and is not moving towards the listener.

BUG=141739

Review URL: https://codereview.chromium.org/301733002

git-svn-id: svn://svn.chromium.org/blink/trunk@175175 bbb929c8-8fbe-4397-9dbb-9b2b20218538