Clarify the initiator steps

[OrKoN]

Issue https://github.com/w3c/webdriver-bidi/issues/698

This PR clarifies the logic of how to compute various initiator types and makes the following changes:

• removes the requestId since it is already included in the network event that contains the initiator.
• adds a URL to hold the referrer for the initators that do not have a stack trace.
• clarifies that the line and column position should be based on the implementation-specific parser metadata that is currently not specified by web standards but usually is available for error reporting and DevTools.
• adds a note clarifying the purpose of the initiator: to give the user the information about how a particular request was triggered, so that the source of the request can be identified.

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[OrKoN]

@juliandescottes @jgraham @whimboo I attempted to improve the initiator steps. There are still a few problems although it brings the steps closer to what is expressed by CDDL types. Problems:

1) I am not quite sure that there referrer would be an URL. The fetch spec mentions that it will be a URL or no-referrer during fetching. CSSOM spec seems to be always setting it without checking for no-referrer. 2) the parser positions are "implementation-defined" since I could not link to any spec. 3) the parser type is extended to cover the css initiators but it is a bit difficult to track what exactly fetch initiators define.

WDYT?