Put timing values of DocumentLoadTiming in a GCed object.
The purpose is that these timing values can outlive the document; and
PerformanceNavigationTiming can collect them.
Also fixed a bug that a NOTREACHED() can be hit if
"PerformanceNavigationTiming::systemEntropy" field is queried on an
iframe that's already been removed.
Bug: 40793421
Change-Id: I4ce6ecb05068aae68176bab6052c5f7e759d902e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6042078
Reviewed-by: Noam Rosenthal nrosenthal@chromium.org
Commit-Queue: Guohui Deng guohuideng@microsoft.com
Reviewed-by: David Baron dbaron@chromium.org
Cr-Commit-Position: refs/heads/main@{#1390482}