Why are concurrent, non-redundant[1] network requests a problem? 

Are sequential requests preferable? 
I assume that on fast connections, this could increase the ETA for the whole set of images.


[1] If the IconCacher is redundantly triggered, it will always query the FaviconService (with favicon::GetFaviconImageForPageURL) but if there is no hit, fetching the actual network request should happen once per URL as ImageFetcherImpl::StartOrQueueNetworkRequest enqueues callback if a request is already running for the given URL. Or am I missing something?

I did not make myself clear: 
I care only about concurrent *redundant* requests (for the same tile/URL). 
I do not prefer sequential. I prefer asking only once for each tile.

You are right that we do not issue multiple network requests.
Still it feels silly to ask the FaviconService 3 times in one second for the very same favicon.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/fc015b3cb9d91a89707e479635fad2cd3695cd1d

commit fc015b3cb9d91a89707e479635fad2cd3695cd1d
Author: jkrcal <jkrcal@chromium.org>
Date: Thu May 18 17:43:21 2017

[NTP Tiles] Avoid duplicate requests from IconCacherImpl

This CL adds recording of requests in-flight so that we do not issue the
same network request twice. The cacher is often triggered both for PS
and for ML several times in a quick succession.

This is a kind of a hack to make experimentation with fetching favicons
for MostLikely tiles in M60 safer (not issuing too many network
requests). This should be removed once the underlying issue in
most_visited_sites is addressed.

BUG=720536

Review-Url: https://codereview.chromium.org/2873403002
Cr-Commit-Position: refs/heads/master@{#472861}

[modify] https://crrev.com/fc015b3cb9d91a89707e479635fad2cd3695cd1d/components/ntp_tiles/icon_cacher_impl.cc
[modify] https://crrev.com/fc015b3cb9d91a89707e479635fad2cd3695cd1d/components/ntp_tiles/icon_cacher_impl.h
[modify] https://crrev.com/fc015b3cb9d91a89707e479635fad2cd3695cd1d/components/ntp_tiles/icon_cacher_impl_unittest.cc

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