This makes these users (which are exceptions within layout, although
low-level enough that it won't matter for font inflation work) call
through to GetMetricsFor explicitly with the correct language, rather
than using the broken nsPresContext::GetMetricsFor and its
charset-detected language.
This improves the correctness of our behavior for 'ch' and 'ex' CSS
units when the font selection (or defaults) are language-dependent. It
should also reduce the number of unique sets of font metrics requested
(which helps nsFontCache effectiveness).
