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* Factor out code that can be used for both chrome/app_shell version from athena/extensions/chrome
* Create AppsClient/AppsDelegate for app_shell version.
* Removed CreateAppsClient as it's no longer necessary.

AthenaNativeAppWindow needs more work in order to work properly on athena. I'll address them in separate bug/CLs.

BUG=410448,410448
TBR=ben@chromium.org

Note: This CL depends on https://codereview.chromium.org/576863003/

Review URL: https://codereview.chromium.org/563083003

Cr-Commit-Position: refs/heads/master@{#295583}