compute pressure: Filter records in PressureObserver

[chromium-wpt-export-bot]

This patch implements the filtering of records in the PressureObserver and not anymore in the platform collector, for the reasons explained below. As described in 1: All observers should be updated at their samplerate unless the previous record is similar to the new record. As discussed in 2, it shouldn't be implemented in the platform collector.

Bug: 1359196 Change-Id: I7b926f391ac119a0d33ec4cf91738540faa039e8

Reviewed-on: https://chromium-review.googlesource.com/3868958 WPT-Export-Revision: afe45804120334b11e234add7e20ce2a1e45c751