Allow always logging tofu detection

This commit allows to log tofu detection always,
even if the rest of the webfonts library is not loaded.

By default this is disabled.

To enable it, set the variable $wgULSTofuLoggingChance
to a number between 0 and 100, indicating the chance of it
getting logged. The idea of the lottery is that even a partial
sample can be useful.

Change-Id: Ia08fe0de348b7eebfa95498a6be58ae41608ba3b
Merge "Allow always logging tofu detection"