Use CSS function ID instead of string literal in SizesCalcParser

This patch changes SizesCalcParser to check CSSParserToken::FunctionId()
instead of string value, so that it's easier to track how calc() is
parsed, and where parser should be extended for min(), max() and
clamp().

Bug: 825895
Change-Id: I327808a31c7790f376e3a9e9c39b77e2d88e0cd0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1679232
Auto-Submit: Xiaocheng Hu <xiaochengh@chromium.org>
Commit-Queue: Emil A Eklund <eae@chromium.org>
Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672914}