Split histograms.xml into multiple smaller xmls

Split the rest of the histograms.xml into multiple smaller xmls. This
makes all histograms are searchable from code search and reduces the
load time to render a histograms.xml file.

This cl hasn't deleted the old histograms.xml yet because there are other
code referencing to it.

Bug: 993193
Change-Id: If813a3aaaf1b60cc4db4860fef31d562a13ef23e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2428796


Commit-Queue: Weilun Shi <sweilun@chromium.org>
Auto-Submit: Weilun Shi <sweilun@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811083}