add uniqueID

BUG=skia:6366

Change-Id: Ie3215a392040be645524a2294d824d953ba3a1b6
Reviewed-on: https://skia-review.googlesource.com/9703
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
diff --git a/tests/VerticesTest.cpp b/tests/VerticesTest.cpp
index 9c433ba..8cf5514 100644
--- a/tests/VerticesTest.cpp
+++ b/tests/VerticesTest.cpp
@@ -79,6 +79,9 @@
             sk_sp<SkData> data = v0->encode();
             sk_sp<SkVertices> v1 = SkVertices::Decode(data->data(), data->size());
 
+            REPORTER_ASSERT(reporter, v0->uniqueID() != 0);
+            REPORTER_ASSERT(reporter, v1->uniqueID() != 0);
+            REPORTER_ASSERT(reporter, v0->uniqueID() != v1->uniqueID());
             REPORTER_ASSERT(reporter, equal(v0.get(), v1.get()));
         }
     }