Bug 315483 – Refactor container-type into a Style value type

• Component: CSS
• Status: RESOLVED FIXED

The container-type property's computed value is currently represented by
the ContainerType enum (normal | size | inline-size), stored as a 2-bit
bitfield in StyleNonInheritedRareData and converted to/from CSS via the
generated 1:1 keyword <-> CSSValueID mapping.

css-conditional-5 redefines the grammar as:
  
container-type = normal | [ [ size | inline-size ] || scroll-state ]
      
https://drafts.csswg.org/css-conditional-5/#container-type
      
The `||` combinator means scroll-state can be combined with size or
inline-size (e.g. "size scroll-state", "inline-size scroll-state"). A
combined value parses to a CSSValuePair of two keywords, and its computed
value must serialize back to two keywords. The generated enum storage
relies on a single keyword <-> single enum value mapping for both parsing
and computed-value serialization, so it cannot represent or round-trip a
combined value. container-type can therefore no longer be a plain enum.

This bug covers the preparatory, non-functional refactor: migrate
container-type to a Style::ContainerType value type
(computed-style-storage-kind "reference") with a custom CSSValueConversion
and switchOn, matching the sibling properties stored in the same struct
(scrollbar-gutter -> Style::ScrollbarGutter, scroll-snap-type -> 
Style::ScrollSnapType). Only normal, size and inline-size are supported in
this change -- there is no behavior change.
  
The scroll-state value itself, including the combined forms and the
feature flag, will be added on top in bug 314082.

Pull request: https://github.com/WebKit/WebKit/pull/65594

[EWS]

Committed 313869@main (d9dde2e0b6ed): <https://commits.webkit.org/313869@main>

Reviewed commits have been landed. Closing PR #65594 and removing active labels.

[Radar WebKit Bug Importer]

<rdar://problem/177921550>