Set initial size in sample_app
Set initial size in sample_app

https://github.com/domokit/mojo/commit/91b812b1af824c8faba2f75d88d31a1532164728
broke sample_app since it threw away the initial viewport size and the
native viewport service's metrics changed callback no longer fires unless
the size changes. This uses the initial size as well.

R=sky@chromium.org

Review URL: https://codereview.chromium.org/885953002

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