Harmonize with Blink Serial API implementation

[reillyeon]

• Blink doesn't implement setOptions() and getInfo(). The semantics of these are unclear but multiple implementation experience here could be instructive.
• Blink expects SerialOptions to be passed to open().
• Blink expects SerialPortRequestOptions to be passed to requestPort().
• Blink does not provide a SerialPort constructor. The constructor provided by the polyfill is useful for manually constructing a SerialPort from a USBDevice which is something the polyfill can support clients doing outside of the specified Serial API.