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Use JS modules instead, since HTML Imports are being removed.

Replace CallJavascriptFunction() with FireWebUIListener(), since within
a JS module context, all variables/functions are private (unlike previous
behavior of being global), and can't be called from the outside

Bug: 937747
Test: Visit chrome://webapks, should no longer see HTML import warnings in DevTools.
Change-Id: I4319d6b78a9a4c370a2f9152e7448e7c3325adf4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1850271
Auto-Submit: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Calder Kitagawa <ckitagawa@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705203}