MDL-39246 JavaScript: store the time the page starts loading.

This helps synchronise time on the client side with time on the server.
If you wait until a js_init_call runs, you might have had to wait an
unknown number of seconds while all the JS loads. Therefore, we capture
the current time as soon as possible in <head>.

This is a bit like adding .jsenabled to <body> as soon as possible, to
reduce visual flicker.
MDL-39246 quiz timer: compute a more accurate stop time.