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There is race when request to set system modal received before
surface window is actually created. This CL keeps system model
property and apply it once window is created.

TEST=Manually, simulate situation for early system modal switch.
     Before fix it crashed with the same stack trace as crash
     reports. After this I observed warning message in log and
     system modal state is working as expected.
BUG=639511

Review-Url: https://codereview.chromium.org/2743053002
Cr-Commit-Position: refs/heads/master@{#456256}