[functions] Add CSSVariableParser::CollectDashedFunctions

This CL adds a function to collect all <dashed-indent> instances
in a CSSParserTokenStream (typically representing a single value).
This will be be used by InspectorCSSAgent to find all relevant
@function rules for a given call to getMatchedStylesForNode.

Bug: 325504770, 392034817
Change-Id: I28a1d243a5e8b24e63fe239577c5d18ec49937a4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6252838
Commit-Queue: Anders Hartvoll Ruud <andruud@chromium.org>
Reviewed-by: Philip Pfaffe <pfaffe@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1423051}