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This is only supported on ChromeOS, so make it conditional and follow
the example of Browser::ShouldShowStaleContentOnEviction():
https://cs.chromium.org/chromium/src/chrome/browser/ui/browser.cc?dr=CSs&q=Browser::ShouldShowStaleContentOnEviction&g=0&l=1453

BUG=1036760

Change-Id: Idcf003483600046a4e64ebe0b681bbdb30b8449a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1992515Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#729809}