Kahakō and ʻokina in Chrome
Here's how I got the kahakō and ʻokina (macron and glottal stop for Hawai'ian) in Chrome.
1) Install https://chrome.google.com/webstore/detail/composekey/iijdllfdmhbmlmnbcohgbfagfibpbgba from the Chrome Web Store (it appears to be made by Google).
2) Click the extension icon and click Options. Then follow the instructions to enable it. I enabled it with Compose Key (US).
3) Add the following lines to the configuration section at the bottom
# Hawaiʻian vowels with kahakō
<Multi_key> <a> <-> : "ā"
<Multi_key> <e> <-> : "ē"
<Multi_key> <i> <-> : "ī"
<Multi_key> <o> <-> : "ō"
<Multi_key> <u> <-> : "ū"
# Hawaiʻan ʻokina
<Multi_key> <q> : "ʻ"
4) Type RightAlt o - to get ō (you don't need to hold down RightAlt). I originally had the - first but it conflicts with the minimize shortcut in ChromeOS. To get an ʻokina, type RightAlt q.
You can add more sequences using the same pattern and replacing the quoted text with any unicode character. To get an arbitrary unicode character in ChromeOS, hit Ctrl+Shift+u, then the four-digit hex number for the character (Google e.g. "unicode o with macron"), then Enter. That's how I generated the above configuration.