Map iframe/embed/video width/height attributes to CSS dimension properties

[nicoburns]

Summary

Follow-up to #607 (companion to the abspos static-position fix in #608).

Per HTML rendering, the width/height attributes on iframe, embed and video map to the corresponding CSS dimension properties, including percentages (<iframe height="50%">). Previously they were only consumed as pixel intrinsic sizes in the replaced layout path, so percentage values were ignored. This adds the mapping in synthesize_presentational_hints_for_legacy_attributes (same pattern as the existing table/svg dimension mappings).

WPT results

Newly passing vs main (no regressions in css-flexbox, css/CSS2/normal-flow, css/CSS2/positioning):

• css/CSS2/positioning/absolute-replaced-height-005/007/012/014/019/026/033.xht

Together with #608 this also fixes inline-replaced-height-005.xht and inline-block-replaced-height-005.xht.

Link to Devin session: https://app.devin.ai/sessions/0f8547b7fbb949e28e9acdbedb1ca8d2
Requested by: @nicoburns

WPT results

10 newly passing, 0 newly failing (net +10).

Full diff (10 changed tests)

+ Fail => Pass css/CSS2/floats-clear/float-replaced-height-005.xht
+ Fail => Pass css/CSS2/normal-flow/inline-block-replaced-height-005.xht
+ Fail => Pass css/CSS2/normal-flow/inline-replaced-height-005.xht
+ Fail => Pass css/CSS2/positioning/absolute-replaced-height-005.xht
+ Fail => Pass css/CSS2/positioning/absolute-replaced-height-007.xht
+ Fail => Pass css/CSS2/positioning/absolute-replaced-height-012.xht
+ Fail => Pass css/CSS2/positioning/absolute-replaced-height-014.xht
+ Fail => Pass css/CSS2/positioning/absolute-replaced-height-019.xht
+ Fail => Pass css/CSS2/positioning/absolute-replaced-height-026.xht
+ Fail => Pass css/CSS2/positioning/absolute-replaced-height-033.xht

_{Generated by the WPT workflow.}

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