AudioParam.exponentialRampToValueAtTime() should throw exception with non-positive values

As given in
https://dvcs.w3.org/hg/audio/raw-file/tip/webaudio/specification.html#methodsandparams-AudioParam,
we need to throw an exception for non-positive target values. This
case is handled here. The case where the previous value is negative is
not handled.

Layout test updated to include a test for this.

BUG=243690

Review URL: https://codereview.chromium.org/157813006

git-svn-id: svn://svn.chromium.org/blink/trunk@166964 bbb929c8-8fbe-4397-9dbb-9b2b20218538