Declare `InputEvent.data` value of `insertLink` should be relative URI if it's specified as relative URI

[masayuki-nakano]

Currently, insertLink''sdata` value is declared as:

the url of the proposed link

This is not clear whether this value can be relative URI or resolved to absolute URI even when relative URI is specified.

I think that the data value of insertLink should be any URI which is specified actually because:

• specified URI may be invalid.
• the document's URI may be data URI.
• web apps wants to resolve the given URI with different URI from the document's URI.