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Add a flag to RunExtensionTest so that an existing event page based
test extension can be loaded as SW based extension. This can be
used to run extenision tests with Service Worker based extensions.

This CL also converts runtime/open_options_page/ test extension to event
page extension and demonstrates that the extension be loaded
as SW extension and the corresponding test
ExtensionApiTest.ChromeRuntimeOpenOptionsPage passes with the flag in
ServiceWorkerBasedBackgroundTest.ChromeRuntimeOpenOptionsPage.

Doc: https://docs.google.com/document/d/1PvXZ7VGRGdmd1s99SFByn9NCGj0GAiBhhPmfjj35ZeI/edit#

Bug: 967899
Change-Id: I8821d1c9fa22795b8fe0f65f1bd073a034e24431
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1626455
Commit-Queue: Istiaque Ahmed <lazyboy@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664062}