Add deletion test, cursor read with writeback to same store.

chrome/test/data/indexeddb/perf_shared.js

Index: chrome/test/data/indexeddb/perf_shared.js
diff --git a/chrome/test/data/indexeddb/perf_shared.js b/chrome/test/data/indexeddb/perf_shared.js
index f335b0b5855b290d422efaa4a7305e481d37394a..a831f65b5f0043b0ec7a5bb94e3398c30b963aa6 100644
--- a/chrome/test/data/indexeddb/perf_shared.js
+++ b/chrome/test/data/indexeddb/perf_shared.js
@@ -332,8 +332,12 @@ function getValuesFromCursor(
     if (cursor) {
       assert(numReadsLeft);
       --numReadsLeft;
-      if (oos) // Put in random order for maximum difficulty.
-        oos.put(cursor.value, Math.random());
+      if (oos)
+        // Put in random order for maximum difficulty.  We add in numKeys just
+        // in case we're writing back to the same store; this way we won't
+        // affect the number of keys available to the cursor, since we're always
+        // outside its range.
+        oos.put(cursor.value, numKeys + Math.random());
       values.push({key: cursor.key, value: cursor.value});
       cursor.continue();
     } else {