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
AudioContext* AudioContext::create(Document& document, ExceptionState& exceptionState)
{
    ASSERT(isMainThread());
    if (s_hardwareContextCount >= MaxHardwareContexts) {
        exceptionState.throwDOMException(
            SyntaxError,
            "number of hardware contexts reached maximum (" + String::number(MaxHardwareContexts) + ").");
        return 0;
    }

    AudioContext* audioContext = new AudioContext(&document);
    audioContext->suspendIfNeeded();
    return audioContext;
}

AudioContext* AudioContext::create(Document& document, ExceptionState& exceptionState)
{
    ASSERT(isMainThread());
    if (s_hardwareContextCount >= MaxHardwareContexts) {
        exceptionState.throwDOMException(
            SyntaxError,
            "number of hardware contexts reached maximum (" + String::number(MaxHardwareContexts) + ").");
        return 0;
    }

    AudioContext* audioContext = new AudioContext(&document);
    audioContext->suspendIfNeeded();
    return audioContext;
}
