Expose contentEncoding in PerformanceResourceTiming

This CL introduce a contentEncoding field to Performance resource timing
object. This field is behind a feature flag.

PR to resource timing specification:
https://github.com/w3c/resource-timing/pull/411
PR to fetch specification:
https://github.com/whatwg/fetch/pull/1796

Bug: 327941462
Change-Id: I70cad190fe658fb3dbf8b401ff8393bc1d0782f0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6098321
Commit-Queue: Guohui Deng <guohuideng@microsoft.com>
Reviewed-by: Noam Rosenthal <nrosenthal@chromium.org>
Reviewed-by: Matthew Denton <mpdenton@chromium.org>
Reviewed-by: Yoav Weiss (@Shopify) <yoavweiss@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1407331}