When loading a canned version of reddit.com, we end up parsing many many
shadow tree style sheets of roughly ~170 KiB text each.
None of them have '\r' or '\f', yet we spend 2-3 ms for each sheet just
looping over and reconstructing the text to see if we need to normalize
any newlines.
This patch makes the common case faster in two ways:
- We use TextCodec::Decoder::to_utf8() instead of process()
This way, we do a one-shot fast validation and conversion to UTF-8,
instead of using the generic code-point-at-a-time callback API.
- We scan for '\r' and '\f' before filtering, and if neither is present,
we simply use the unfiltered string.
With these changes, we now spend 0 ms in the filtering function for the
vast majority of style sheets I've seen so far.
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