Use .equals() for string comparisons in ModelType.

This fixes a FindBugs warning.

NOTRY=true
BUG=none

Review URL: https://codereview.chromium.org/517453003

Cr-Commit-Position: refs/heads/master@{#292364}

sync/android/java/src/org/chromium/sync/internal_api/pub/base/ModelType.java

@@ -90,7 +90,7 @@ public enum ModelType {
     private final boolean mNonInvalidationType;
 
     ModelType(String modelType, boolean nonInvalidationType) {
-        assert nonInvalidationType || modelType == toString();
+        assert nonInvalidationType || modelType.equals(toString());
         mModelType = modelType;
         mNonInvalidationType = nonInvalidationType;
     }