To enable it (as of August 2017), you need the following steps:
1. Use Chrome Canary
2. Enable experiments: chrome://flags/#enable-devtools-experiments (copy and paste this internal link into Canary)
3. From devtools, open settings (F1)
4. Open the experiments panel
5. Hit shift seven times (no, I'm not kidding)
6. Check the "Color contrast ratio line in color picker"