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The background page only existed to support BT Classic flow, which is
now gone.

R=tengs@chromium.org

Bug: none
Test: none
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I4af74256299656a119507a209b875a213226fadd
Reviewed-on: https://chromium-review.googlesource.com/964909Reviewed-by: Tim Song <tengs@chromium.org>
Commit-Queue: James Hawkins <jhawkins@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543522}