Output silence if the MediaElementAudioSourceNode has a different origin

See http://webaudio.github.io/web-audio-api/#security-with-mediaelementaudiosourcenode-and-cross-origin-resources

Two new tests added for the same origin and a cross origin source.

BUG=313939

Review URL: https://codereview.chromium.org/520433002

git-svn-id: svn://svn.chromium.org/blink/trunk@189527 bbb929c8-8fbe-4397-9dbb-9b2b20218538
ChannelMergerNode* AudioContext::createChannelMerger(ExceptionState& exceptionState)
{
    const unsigned ChannelMergerDefaultNumberOfInputs = 6;
    return createChannelMerger(ChannelMergerDefaultNumberOfInputs, exceptionState);
}

ChannelMergerNode* AudioContext::createChannelMerger(ExceptionState& exceptionState)
{
    const unsigned ChannelMergerDefaultNumberOfInputs = 6;
    return createChannelMerger(ChannelMergerDefaultNumberOfInputs, exceptionState);
}
