When we generate LCP entries, we store the precision-reduced
timestamps which are convenient for later usages, but not very
performant. This patch makes it instead of storing the
precision-reduced timestamps, store the raw timestamps, and only
reduce the precision when they're being used by user code.
Also, this patch changes how we compute precision-reduced timestamps for LCP.
Currently, we effectively use `ReduceTimePrecisionAsMSecsRFPOnly` to
reduce precision which only work in RFP mode. This patch changes it
to `ReduceTimePrecisionAsMSecs` to make it work in 'Normal' mode (i.e.
non-COOP/COEP pages).
Differential Revision: https://phabricator.services.mozilla.com/D200200
e506e5cfea