SharedArrayBuffer enabled for cross-origin-isolated GLib ports

Component: JSC WebAssembly | 104506c

SharedArrayBuffer (SAB) lets multiple threads/workers share backing memory without copies — exactly the fine-grained shared state that enables cache-timing and speculative side channels — so browsers gate it behind cross-origin isolation (COOP+COEP). This commit sets JSC_useSharedArrayBuffer for GLib-backed WebKit (WPE, GTK) when the origin is cross-origin isolated, unskipping 35 COI WPT tests and dropping four stale baselines.

Significance

SAB, a load-bearing primitive for Spectre-class timing attacks, and its associated risk surface now reach WPE/GTK/Android for the first time. The flag is set in WebProcess::initializeWebProcess just before VM startup; per the commit message, Cocoa's process signal-handling may need the flag set earlier, but GLib/Android processes are service-spawned without that early-init constraint, so the later setting is deliberate and sufficient.

Audit directions

Narrow: confirm the COOP/COEP gating actually fronts the flag on every WPE/GTK entry — a page that acquires SAB without isolation is the failure. Wider: any port-specific flag set in initializeWebProcess relative to VM startup is timing-sensitive; audit other side-channel-relevant JSC flags for the same late-set pattern. The tell: a JSC_use* flag toggled in initializeWebProcess with a comment about process-init ordering.

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