metrics: Add enum values for IPP-USB scanning backend

The ippusb backend is the same as the airscan backend, but used over USB
instead of the network.  Add enum values corresponding to the same
manufacturer list that we track for the airscan backend.
crrev.com/c/2533675 adds the same values on the Chrome OS side.

Bug: b:153544296
Bug: b:160472550
Change-Id: I0bc253f58a16231c5b995b3f8aac0c89273941bd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2533795Reviewed-by: Jesse Schettler <jschettler@chromium.org>
Commit-Queue: Benjamin Gordon <bmgordon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826678}

tools/metrics/histograms/enums.xml

@@ -17724,6 +17724,18 @@ Called by update_document_policy_enum.py.-->
   <int value="92" label="Umax1220U"/>
   <int value="93" label="UmaxPp"/>
   <int value="94" label="XeroxMfp"/>
+  <int value="95" label="IppUsbBrother"/>
+  <int value="96" label="IppUsbCanon"/>
+  <int value="97" label="IppUsbEpson"/>
+  <int value="98" label="IppUsbHp"/>
+  <int value="99" label="IppUsbKodak"/>
+  <int value="100" label="IppUsbKonicaMinolta"/>
+  <int value="101" label="IppUsbKyocera"/>
+  <int value="102" label="IppUsbLexmark"/>
+  <int value="103" label="IppUsbOther"/>
+  <int value="104" label="IppUsbRicoh"/>
+  <int value="105" label="IppUsbSamsung"/>
+  <int value="106" label="IppUsbXerox"/>
 </enum>
 
 <enum name="DocumentStateForDeferredLoading">