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This change removes PageLoad.Clients.DataReductionProxy.LoFiOn.* and
adds PageLoad.Clients.Previews.LitePages. The key difference in these
two histograms is the earlier triggers for all pages that use server
LoFi or server LitePages, whereas the new UMA will only trigger for
pages that use server LitePages.

Bug: 720035
Change-Id: I48c6f349aa5e1a6dfc2e261729ca8d3d624cb08d
Reviewed-on: https://chromium-review.googlesource.com/1060098Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Commit-Queue: Ryan Sturm <ryansturm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558884}