Fix GetUtfLength to add offset to char array data.

GetUtfLength isn't used by much, so I made ToModifiedUtf8 use it so it
can be exercised more. Also added a test to make sure it's working.

Change-Id: I9d569642aaf2313cc70a2e22f631aec138e2e71b
diff --git a/src/object_test.cc b/src/object_test.cc
index 80ed0bb..bf07aa3 100644
--- a/src/object_test.cc
+++ b/src/object_test.cc
@@ -293,6 +293,18 @@
   EXPECT_FALSE(empty->Equals("a"));
 }
 
+TEST_F(ObjectTest, StringLength) {
+  SirtRef<String> string(String::AllocFromModifiedUtf8("android"));
+  EXPECT_EQ(string->GetLength(), 7);
+  EXPECT_EQ(string->GetUtfLength(), 7);
+
+  string->SetOffset(2);
+  string->SetCount(5);
+  EXPECT_TRUE(string->Equals("droid"));
+  EXPECT_EQ(string->GetLength(), 5);
+  EXPECT_EQ(string->GetUtfLength(), 5);
+}
+
 TEST_F(ObjectTest, DescriptorCompare) {
   ClassLinker* linker = class_linker_;