Fix LoFi preview type histogram suffix description.

The current description of the preview type histogram suffix "LoFi" says
that it only refers to server-side LoFi previews, but in reality it
actually refers to both server-side and client-side LoFi previews, so
this CL updates that description.

Change-Id: I4f933186c39adcbb6bc68805ad64f1da2e934904
Reviewed-on: https://chromium-review.googlesource.com/1107200Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Scott Little <sclittle@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568949}

tools/metrics/histograms/histograms.xml

@@ -121444,7 +121444,7 @@ uploading your change for review.
 <histogram_suffixes name="Previews.Types" separator=".">
   <suffix name="AMPRedirection" label="AMP Redirection previews"/>
   <suffix name="LitePage" label="Lite page previews"/>
-  <suffix name="LoFi" label="Server LoFi previews"/>
+  <suffix name="LoFi" label="LoFi previews"/>
   <suffix name="NoScript" label="NoScript previews"/>
   <suffix name="Offline" label="Offline previews"/>
   <affected-histogram name="Previews.EligibilityReason"/>