Text substitution does not work with Slack on Yosemite

Activate the "Text Replacement" option in the edit menu.

Even after following the instructions in the accepted answer, this still did not work for me until I followed these instructions. Basically, run the following in a terminal:

defaults write -g WebAutomaticTextReplacementEnabled -bool true

or to only change Slack's behavior (and not pollute your global domain):

defaults write com.tinyspeck.slackmacgap WebAutomaticTextReplacementEnabled -bool true

So, it seems that Text Substitutions have not been implemented in Chrome so I assume that is the underlying problem I'm having with Slack.