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This CL changes the way DiceInterceptedSessionStartupHelper adds the
account on the web.
Previously it was mostly waiting for the reconcilor to trigger, and
making sure that the process is not stuck (by forcing calls to list
accounts).

The new behavior is more proactive: when the profile is new, it blocks
the reconcilor and forces a call to multilogin directly.
The advantages are:
- it is easier to control and hopefully it should be less brittle.
- it should be faster:
  * the previous flow was doing an extra /ListAccounts call after the
    reconciliation, which is now removed
  * the previous flow was using MergeSession while the new flow uses
    multilogin, which should be faster on desktop (no need to fetch an
    access token)

When the profile is not new, it's not possible to simply use multilogin,
as there may already be accounts in the profile. The reconcilor is still
used in this case, as it has the logic to handle this.

Note: it would be possible to use MergeSession instead of Multilogin, as
this would work for all profiles (including existing profiles). However
MergeSession is now discouraged and thus we avoid adding a new usage
here.

This CL also improves histograms.

Bug: 1151313
Change-Id: I4871eb123eb37081595164bd5280adf2b0b1290b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2600909
Commit-Queue: David Roger <droger@chromium.org>
Commit-Queue: Alex Ilin <alexilin@chromium.org>
Auto-Submit: David Roger <droger@chromium.org>
Reviewed-by: Alex Ilin <alexilin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840690}