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This CL adds support for Service Worker-based extensions to the
ExtensionBuilder class. It also renames the SetBackgroundPage()
member function to SetBackgroundContext().

Bug: 853378
Change-Id: I30018b46aafefdfb8e6e450101b86f489669cc36
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1719376
Commit-Queue: David Bertoni <dbertoni@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#681133}