Cached display list command sequences used to carry their own
DisplayListResourceStorage. That kept resource ID sets and referenced
fonts, images, video frames, and nested display lists alive on every
cached phase, even though the command bytes already contain enough
information to discover those references when they are needed.
This makes cached command sequences store only command bytes. Resource
references are collected transiently from those bytes when a cache entry
is installed or invalidated. The navigable's central display list
resource storage now keeps cache reference counts, so compositor pruning
retains resources used by live cached commands without duplicating
storage in each sequence.
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