static getMediaKeySystemAccess(videoCodec, audioCodec, keySystem) {
    const DrmUtils = shaka.drm.DrmUtils;
    const key = DrmUtils.generateKeySystemCacheKey_(
        videoCodec, audioCodec, keySystem);
    return DrmUtils.memoizedMediaKeySystemAccessRequests_.get(key) || null;
  }
Get MediaKeySystemAccess object for following attributes.

@param {string} videoCodec
@param {string} audioCodec
@param {string} keySystem
@return {?MediaKeySystemAccess}