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setTargetAtTime should allow a time constant of 0.  This means the
output value jumps instantaneously to the target value at the start
time.

Spec: https://webaudio.github.io/web-audio-api/#widl-AudioParam-setTargetAtTime-AudioParam-float-target-double-startTime-float-timeConstant

BUG=630869
TEST=audioparam-setTarget-timeConstant-0.html,
     audiparam-exceptional-values.html

Remove test for timeConstant=0.

Review-Url: https://codereview.chromium.org/2426893003
Cr-Commit-Position: refs/heads/master@{#437614}