Protect some definitions in about_flags.cc with #defined(USE_ASH)

kAshMaterialDesignChoices, kAshMaxWindowsToUseMaskInOverviewChoices
and kAshMaxWindowsToUseShapeInOverviewChoices are currently only defined
when OS_CHROMEOS is true but they are actually only used when USE_ASH is
true. This CL protect the definitions with USE_ASH instead of
OS_CHROMEOS.

BUG=None

Review-Url: https://codereview.chromium.org/2358573002
Cr-Commit-Position: refs/heads/master@{#424125}
1 file changed