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This is required to show the status area widget in mojo:ash.

* Initialize various service singletons in WindowManagerApplication
* Add ash strings to the window manager pak file

BUG=619636
TEST=ash_unittests, mash_unittests

Review-Url: https://codereview.chromium.org/2155743003
Cr-Commit-Position: refs/heads/master@{#406126}