How to reproduce:
1. Download attached project file and open "Scene" Scene
2. Press Build & Run
3. In browser select "Allow WebCam"

Actual result: Error "Cannot read property 'now' of undefined" in WebGL build unless exit from C# Start() is delayed. This error appears after "allow webcam" is selected. Emptying the cache and reloading the page doesn't encounter this error. Another way of avoiding the error is to add a call to Debug.Log to the end of the Start method of a C# script attached to a game object -- this suggests there is a race condition about when the code that follows Start begins execution.

Reproduced with: 5.6.4p2, 2017.1.2p3, 2017.2.0p2, 2017.3.0b9, 2018.1.0a4