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It is unnecessary since all tests did load chrome://extensions as
|browsePreload| anyway, but most importantly this causes the extensions
page to load twice during optimized builds, which leads to runtime errors.

This is in preparation of turning on optimize_webui for MD Extensions.

Bug: 764126
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I5e8b903bd4b5589af26fea6b3a25290d56188a7a
Reviewed-on: https://chromium-review.googlesource.com/703861
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Hector Carmona <hcarmona@chromium.org>
Cr-Commit-Position: refs/heads/master@{#506964}