Avoid AtomicString implicit conversions in core/css/

This is a preparation to add `explicit` to AtomicString constructors.

* Just wrap literals with AtomicString() in many cases
* `name` in CascadeLayer::ToStringInternal() and the return type of
  CSSNumericValueType::BaseTypeToString() don't need to be AtomicString.

This CL should have no behavior changes.

Bug: 1444094
Change-Id: I8f6b435fae7f358a1a2a3e83bfee3dd83fab5aa8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4561005
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Auto-Submit: Kent Tamura <tkent@chromium.org>
Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1148596}