Currently tab capture from an incognito window fails for MR: it captures a tab from the original (non-incognito) profile.

It looks like the fix is to have the tabCapture API search incognito contexts when looking for the tab to capture.

https://code.google.com/p/chromium/codesearch#chromium/src/chrome/browser/extensions/api/tab_capture/tab_capture_api.cc&l=206

We can whitelist this if need be, but then other non-split-mode extensions will run into the same bug.