Make setTarget followed by linear or exponential ramp continuous.

When setTarget is followed by a linear or exponential ramp, the
current behavior produces discontinuous curves.  In this case, make
the curve continuous by inserting a setValue to freeze the curve at
the last value of the setTarget.  This establishes the starting point
for the following linear and exponential ramp to make it continuous.

WebAudio issue: https://github.com/WebAudio/web-audio-api/issues/652
Spec proposal: https://github.com/WebAudio/web-audio-api/pull/665

BUG=564157
TEST=audioparam-setTargetAtTime-continuous.html

Review-Url: https://codereview.chromium.org/1485003002
Cr-Commit-Position: refs/heads/master@{#391597}
3 files changed