Add new tflite 2.3.0 enum

This matches the enum checked into google3

Bug: 1146151
Change-Id: I934ae52656e765992c755733f4108773d6446f0e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2552760
Commit-Queue: Sophie Chang <sophiechang@chromium.org>
Commit-Queue: Michael Crouse <mcrouse@chromium.org>
Auto-Submit: Sophie Chang <sophiechang@chromium.org>
Reviewed-by: Michael Crouse <mcrouse@chromium.org>
Cr-Commit-Position: refs/heads/master@{#829851}

components/optimization_guide/proto/models.proto

@@ -247,6 +247,8 @@ enum ModelType {
   MODEL_TYPE_UNKNOWN = 0;
   // A decision tree.
   MODEL_TYPE_DECISION_TREE = 1;
+  // A model using only operations that are supported by TensorflowLite 2.3.0.
+  MODEL_TYPE_TFLITE_2_3_0 = 2;
 }
 
 // A set of model features and the host that it applies to.