Detect tofu before applying any default fonts

To detect whether the client can render a given string,
use a tofu detection algorithm.

If the user has an explicit font preference, do not override it.

If the user has no preference and there is a default font for a
language, see if that language can be rendered at the user's browser
using tofu detection.

If tofu is detected, apply the font.

If the element has 'autonym' class, apply Autonym font only when
that language has tofu.

Change-Id: Ib9ee9497e6bcfa7eb86f7d264e0980d8880d6b70
Merge "Detect tofu before applying any default fonts"