RTCDataChannel.send steps for bufferedAmount

[lgrahl]

Step 3 states:

Let data be a byte buffer that represents the result of encoding the method's argument as UTF-8 and increase the value of the [[BufferedAmount]] slot with the length of data.

But step 4 raises TypeError in case the message is too large to be sent (> maxMessageSize). So, [[BufferedAmount]] should not be increased before that.

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Two more things:

• Step 4 should explicitly mention that size of data means the size in bytes.
• A typo in two places: slot with the the length.

[stefhak]

@lgrahl please provide a PR