Support redundant response start in PageTimingMetricsSender.

PageTimingMetricsSender::DidStartResponse() can be invoked multiple
times with the same resource id, despite the long-standing DCHECK
suggesting that it doesn't happen (see bug for repro). Prior to
crrev.com/c/4321469, when that happened, the `PageResourceDataUse`
instance already in `page_resource_data_use_` was reused (emplace()
no-ops if the element already exist). Since crrev.com/c/4321469,
the existing `PageResourceDataUse` was deleted and replaced with
a new one. This could result in a dangling pointer in
`modified_resources_`. This CL solves the issue by making
`InsertPageResourceDataUse()` no-op if there is already an entry
with the given id in `page_resource_data_use_`.

Bug: 1504430
Change-Id: Ibd2108006e6ea284f12a722cf16ff72da58e6cea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5056238
Auto-Submit: Francois Pierre Doray <fdoray@chromium.org>
Reviewed-by: Annie Sullivan <sullivan@chromium.org>
Commit-Queue: Annie Sullivan <sullivan@chromium.org>
Reviewed-by: Hao Liu <haoliuk@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1233966}