metrics: Add FilesystemVerifierError metric.

In https://chromium-review.googlesource.com/#/c/267554/ we added a new
error code to ChromeOS update engine. This adds FilesystemVerifierError
to histograms.xml.

BUG=None
TEST=unit tests, pretty_print.py.
R=deymo@chromium.org

Review URL: https://codereview.chromium.org/1108843005

Cr-Commit-Position: refs/heads/master@{#327222}

tools/metrics/histograms/histograms.xml

@@ -63757,6 +63757,7 @@ To add a new entry, add it with any value and run test to compute valid value.
   <int value="44" label="kErrorCodeUnsupportedMajorPayloadVersion"/>
   <int value="45" label="kErrorCodeUnsupportedMinorPayloadVersion"/>
   <int value="46" label="kErrorCodeOmahaRequestXMLHasEntityDecl"/>
+  <int value="47" label="kErrorCodeFilesystemVerifierError"/>
 </enum>
 
 <enum name="UpdateEngineInstallDateProvisioningSource" type="int">