Make nsRuleNode's use of font metrics for ch and ex units pass the correct...

Make nsRuleNode's use of font metrics for ch and ex units pass the correct language.  (Bug 678671, patch 3)  r=roc

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).