[PA] Move Metadata outside of GigaCage (3/5)

Revert `template <MetadataKind> SlotSpanMetadata`.

DesignDoc:
https://docs.google.com/document/d/1Mn-qRAWuDhS_gRL_OG2TYL-EW5D8yljaCyM40S0ocVc/edit?tab=t.0

Bug: crbug.com/40238514
Change-Id: I2d5d4642d754bca97edf2d04d263938ce620cf8e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6758537
Reviewed-by: Sergei Glazunov <[email protected]>
Reviewed-by: Yuki Shiino <[email protected]>
Reviewed-by: Keishi Hattori <[email protected]>
Commit-Queue: Takashi Sakamoto <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1490549}
NOKEYCHECK=True
GitOrigin-RevId: 133753cd6a1c5a028f8fad9834204260975a859a
1 file changed