[resource-timing] Reduce HTMLObjectElement RT entry flakiness

Several of our RT tests were suffering from flakiness, which turned out
to be implementation flakiness [1]. Apparently, RT entries weren't
firing in some cases, based on some race related to the mojo pipe
between URLLoader and ObjectNavigationFallbackBodyLoader.

This CL fixes that flakiness, by not relying on OnDataCompleted calls
from the mojo layer, and instead count the number of bytes received
from the response, and finalize the response when they match the number
of bytes the response contained.

[1] https://docs.google.com/document/d/1pD640qVkefcnen5yeqd-ZGTrygL_d4jG3ktjq_T78V0/edit#

Bug: 1408635
Change-Id: I351493593326dd7c637be55d7c98a19c4159e825
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4176867
Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Commit-Queue: Yoav Weiss <yoavweiss@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1095596}