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Polymer must be configured before loading. Several files imported polymer.html and not polymer_config.html. When these files are opened in a browser test, either:
* polymer_config.html is never imported, so the test is run under shady DOM, or
* the file indirectly imports polymer_config.html via its dependencies, causing the configuration to happen after Polymer is already loaded (an error)

Using polymer_config.html in place of polymer.html will eliminate this problem, meaning one less error to debug when writing tests. Since every custom Polymer element should import polymer_config.html, we can just place the actual Polymer import in there.

This CL applies this to MD Downloads, MD Settings, and cr_elements.

Review URL: https://codereview.chromium.org/1767913002

Cr-Commit-Position: refs/heads/master@{#381036}