Disable ash configuring display when running mash.

When running ash inside of mash_shell on a device, DisplayConfigurator
will request a NativeDisplayDelegate from Ozone. Ozone is initialized
only in the mus process, not in the ash process, so ash crashes at this
point.

Add accessor for configure_display_ to avoid crashing. The default
display size can be set via command line with --ash-host-window-bounds
flag until a better solution exists.

BUG=590096

Review URL: https://codereview.chromium.org/1782093003

Cr-Commit-Position: refs/heads/master@{#380663}
void Shell::OnOverviewModeEnded() {
  FOR_EACH_OBSERVER(ShellObserver, observers_, OnOverviewModeEnded());
}

void Shell::OnOverviewModeEnded() {
  FOR_EACH_OBSERVER(ShellObserver, observers_, OnOverviewModeEnded());
}
