Tested the issue on chrome reported version# 68.0.3440.106 using Mac 10.13.6 with steps mentioned below:
1) Launched chrome reported version and Opened Devtools>Console
2) Executed indexedDB.open('test').onupgradeneeded = event => {
    const db = event.target.result;
    const objectStore = db.createObjectStore('test');
    objectStore.createIndex('id', ['a', 'b'], {'unique': true});
    objectStore.transaction.oncomplete = _ => {
        indexedDB.open('test').onsuccess = event => event.target.result
            .transaction('test', 'readwrite')
            .objectStore('test')
            .add(1, ['a', 'b']);
    }
} 
3) Navigated to Applications tab and selected IndexedDB folder, tried deleting the instance generated, but unable to delete it

@Reporter: Please find the attached screencast for your reference and let us know if you are also pointing the same which we have captured in the screencast and provide your feedback on it which helps us in further triaging it.

Thanks!

Deleted: 879299.mp4 2.0 MB

	879299.mp4 2.0 MB View Download

Confirm - on your screencast the same defect as in my attached gif.

Thank you for providing more feedback. Adding the requester to the cc list.

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

Able to reproduce the issue on reported chrome version 68.0.3440.106 also on latest chrome 71.0.3542.0 using Mac 10.13.6, Ubuntu 17.10 and Windows 10.  

Same behavior is seen on M-65(65.0.3325.181) hence considering it as non-regression and marking it as Untriaged.
NOTE: UI updated from M-65(65.0.3325.181) before M-65 delete option not available.

Thanks!

Assigning to Daniel because he knows IndexedDB and can prioritize as needed.

Since this is a non-regression and a fairly specific use-case, bumping priority down to P3.

This would be a good first bug.

Marking under devtools as well as the fix will probably involve DevTools code.

Thanks for the report and the easy repro!

I'd like to have a try.

One suggestion for a starting point- if you build chrome with your gn args including "debug_devtools = true", then you can 'Inspect' the DevTools window with another devtools window. This should show whatever error is happening on delete.

Also:
https://gist.github.com/paulirish/2d84a6db1b41b4020685

  async _deleteButtonClicked(node) {
    if (!node) {
      node = this._dataGrid.selectedNode;
      if (!node)
        return;
    }
    const key = /** @type {!SDK.RemoteObject} */ (this._isIndex ? node.data.primaryKey : node.data.key);
    const keyValue = /** @type {!Array<?>|!Date|number|string} */ (key.value);
    await this._model.deleteEntries(this._databaseId, this._objectStore.name, window.IDBKeyRange.only(keyValue));
    this._refreshObjectStoreCallback();
  }

The key.value is undefined in this case.
I'm not sure how to construct/get the Array from RemoteObjectImpl.

From poking around at https://cs.chromium.org/chromium/src/third_party/blink/renderer/devtools/front_end/sdk/RemoteObject.js it looks like you'll need to reconstruct the array locally, using SDK.RemoteArray.objectAsArray(key) and then e.g. map() to convert child values to keys. 

Not sure what layer this should live in. Maybe eostroukhov@ can advise?

As the child values can also be compound objects, it seems that it's not easy to support all kinds of array/objects.

Is there an easy way to reconstruct an object from RemoteObject?

Hrmmm, maybe pfeldman@ can point us towards someone who can advise?

I'm not sure whether there is a requirement to support 'compound index' in inspector. Pls someone else check this bug.

An additional test case:
```
indexedDB.open('test').onupgradeneeded = event => {
    const db = event.target.result;
    const objectStore = db.createObjectStore('test');
    objectStore.createIndex('id', ['a', 'b'], {'unique': true});
    objectStore.transaction.oncomplete = _ => {
        indexedDB.open('test').onsuccess = event => event.target.result
            .transaction('test', 'readwrite')
            .objectStore('test')
            .add(1, ['a', ['a', 1]]); // nested non-homogeneous array as key
    }
}
```