Signal exceptions if start/stop is called out-of-order or too many times.

For AudioBufferSourceNodes and OscillatorNodes, start and stop must
signal errors when called more than once. An error must be signaled if
stop is not called after start.

The implementation of the legacy noteOn and noteOff methods is now
merged into start and stop. This is not backwards compatible since
noteOn and noteOff previously did allow multiple calls.

Tests have been added for these exceptions.

BUG=306139

Review URL: https://codereview.chromium.org/54173005

git-svn-id: svn://svn.chromium.org/blink/trunk@161113 bbb929c8-8fbe-4397-9dbb-9b2b20218538