Call _notifyMountStatusChanged() on App startup
Call _notifyMountStatusChanged() on App startup

A recent change caused my scrolling app to fail because
didMount() was no longer called on my Scrollable subclass
at startup time.

I haven't been able to sort out how this used to work.
This CL is a hack to make it work again.

R=rafaelw@chromium.org

Review URL: https://codereview.chromium.org/1019973003

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