metrics: Update UpdateEngineErrorCode with new error code.

We just added a new error code to ChromeOS's update_engine so also
update histograms.xml.

BUG=chromium:406546
TEST=Validated the xml, pretty print succeeded.

Review URL: https://codereview.chromium.org/514003002

Cr-Commit-Position: refs/heads/master@{#292279}

tools/metrics/histograms/histograms.xml

@@ -50800,6 +50800,7 @@ To add a new entry, add it with any value and run test to compute valid value.
   <int value="43" label="kErrorCodePostinstallFirmwareRONotUpdatable"/>
   <int value="44" label="kErrorCodeUnsupportedMajorPayloadVersion"/>
   <int value="45" label="kErrorCodeUnsupportedMinorPayloadVersion"/>
+  <int value="46" label="kErrorCodeOmahaRequestXMLHasEntityDecl"/>
 </enum>
 
 <enum name="UpdateEngineInstallDateProvisioningSource" type="int">