Add new values to SmartDimModelResult enum

These were added in http://crrev.com/c/1352824 (model_impl.h) but we
didn't add them to enum.xml yet.

Bug: 933200
Change-Id: If18be5107be2124797f72f0f290c0666e61a3c9a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1496122Reviewed-by: Jia Meng <jiameng@chromium.org>
Commit-Queue: Andrew Moylan <amoylan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638496}

tools/metrics/histograms/enums.xml

@@ -43548,6 +43548,13 @@ Called by update_net_trust_anchors.py.-->
     Other error unrelated to preprocessor. This signals a bug in the feature
     generation process.
   </int>
+  <int value="4" label="Mismatched feature size error">
+    The ExampleProprocessor returned a feature vector with an incorrect size.
+  </int>
+  <int value="5" label="Missing ML Service connection error">
+    The ML Service connection had not been initialized at the time it was
+    needed.
+  </int>
 </enum>
 
 <enum name="PowerMLSmartDimParameterResult">