[Extensions] Context menu tests cleanup.

Change all context meny browser tests to use "scripts" instead of "page"
for "background" to match the service worker tests. We will eventually
unify these two sets of tests by generating a manifest file on the fly
with the appropriate key to run either a background page-based or service
worker-based test.

All uses of window.onload were removed, since service workers don't have
a window object.

Bug: 939664
Change-Id: I5d793f19c3bf939a0c151b4833e0128c964bc6d4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1521710
Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Commit-Queue: David Bertoni <dbertoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#641996}
29 files changed