Google Chrome	65.0.3325.209 (Official Build) (64-bit)
Platform	10323.67.0 (Official Build) stable-channel panther

What steps will reproduce the problem?

(1) In a chrome.input.ime.onKeyEvent listener, log `key` property of incoming events to the console.

(2) In the manifest.json for the extension, configure it to use a layout with dead keys (such as "us(intl)"). For example:

    {
      "name": "Compose Key (US International)",
      "type": "ime",
      "id": "compose_key_us_intl",
      "description": "…",
      "language": "en-US",
      "layouts": ["us(intl)"]
    },

(3) Install the IME extension and activate its input method.

(4) Press one of the keys that results in a dead key.


What is the expected result?
Per https://www.w3.org/TR/uievents-key/, the `key` attribute of the event should be set to the string 'Dead'.


What happens instead?
The `key` attribute is set to the empty string, with no other indication that it is a dead key.


(See also https://crbug.com/830854.)