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Also bring back |extraLibraries| override in welcome_browsertest.js (along with
a new explanatory comment), which was removed in a previous CL.

Bug: None
Change-Id: Ib3ed274af8519bbc541eeb435c6cc37abfa3611f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1860149
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Auto-Submit: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705815}