desktop-pwas: Clean up and add new location bar tests

Changes tests to use embedded test servers so that URLs can be loaded
successfully. This deflakes tests and allows us to test more cases e.g.
pages with mixed content, pages with invalid certificates etc.

Also adds tests for mixed content and invalid certificates.

Based on https://crrev.com/c/923226 by mgiuca.

Bug: 825713, 814400
Change-Id: I8603f3e6340eebbbc2faed594825745aed10bb0a
Reviewed-on: https://chromium-review.googlesource.com/994553Reviewed-by: Ben Wells <benwells@chromium.org>
Reviewed-by: Matt Giuca <mgiuca@chromium.org>
Commit-Queue: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548718}

chrome/test/data/extensions/https_app_no_www/manifest.json

-{
-  "name": "App Test",
-  "version": "1",
-  "manifest_version": 2,
-  "app": {
-    "launch": {
-      "web_url": "https://example.com/"
-    }
-  }
-}