devtools: Fix id collisions by using incrementing counters (#35971)

Devtools clients need a `browserId`, `browsingContextID`, and
`outerWindowID`, which correspond to WebViewId, BrowsingContextId, and
PipelineId in Servo. These u32 values were previously derived from our
sharded (u32,u32) id values by taking only the `index` (second u32) and
ignoring the `namespace_id` (first u32), leading to collisions.

This patch fixes that by mapping those Servo ids to sequential u32
values.

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Signed-off-by: Delan Azabani <dazabani@igalia.com>

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