Find which Chrome Tab is using Significant Energy
Right click the table headers (App Name, Energy Impact, etc.) and check Process ID. Now you can see the PIDs associated with that Energy Impact values. 

Next, click the 'customize/options' button in Chrome (it's the triple horizontal stripes button next to the URL bar) and go to More Tools > Task Manager, which brings up Chrome's internal task manager. In there you'll find the different Chrome threads (including running extensions and open tabs) with their names and PIDs. 

You can then cross reference the PIDs between Chrome's task manager and Yosemite's Activity Monitor.