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This parameter is not really needed since position is always the count
of current Infobars, meaning it was always adding the Infobars in the
last position:
https://cs.chromium.org/chromium/src/components/infobars/core/
infobar_container.cc?type=cs&q=OnInfoBarAdded&sq=package:chromium&g=0&l=98

Also, since we now have 2 InfobarContainers a single one of them can't
keep track of how many Infobars are presented, for that reason the
DCHECK on legacy_infobar_container is no longer relevant.

Bug: 911864
Change-Id: I47909d40af15f40d80f822de31f865fb732c8047
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1574289
Commit-Queue: Sergio Collazos <sczs@chromium.org>
Reviewed-by: Chris Lu <thegreenfrog@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652575}