Fix crash when creating an ImageBitmap from an invalid canvas

BUG=354356

Review URL: https://codereview.chromium.org/211313003

git-svn-id: svn://svn.chromium.org/blink/trunk@169973 bbb929c8-8fbe-4397-9dbb-9b2b20218538
void ImageBitmapFactories::ImageBitmapLoader::rejectPromise()
{
    v8::Isolate* isolate = m_scriptState->isolate();
    ScriptScope scope(m_scriptState);
    m_resolver->reject(ScriptValue(v8::Null(isolate), isolate));
    m_factory->didFinishLoading(this);
}

void ImageBitmapFactories::ImageBitmapLoader::rejectPromise()
{
    v8::Isolate* isolate = m_scriptState->isolate();
    ScriptScope scope(m_scriptState);
    m_resolver->reject(ScriptValue(v8::Null(isolate), isolate));
    m_factory->didFinishLoading(this);
}
