Use ArrayInstance.getLength method.

Now that perflib provides it as a better way to get the length of an
array instance.

Change-Id: I6bf8bb5541229e16f36fa8ede0a07cd0eb7b931d
Test: m ahat-test
Bug: 30481345
diff --git a/tools/ahat/src/InstanceUtils.java b/tools/ahat/src/InstanceUtils.java
index 3cdb40c..8769d11 100644
--- a/tools/ahat/src/InstanceUtils.java
+++ b/tools/ahat/src/InstanceUtils.java
@@ -95,9 +95,7 @@
       return null;
     }
 
-    // TODO: When perflib provides a better way to get the length of the
-    // array, we should use that here.
-    int numChars = chars.getValues().length;
+    int numChars = chars.getLength();
     int count = getIntField(inst, "count", numChars);
     if (count == 0) {
       return "";