Add support for gx font variation axes.

This adds an 'axis' child element to the 'font' element. The 'axis'
element has attributes 'tag' (a four byte identifier) and 'stylevalue'
(a float value) to the parser. This also modifies reading the font file
name in a backwards compatible fashion by using only the direct #text
children of the 'font' element. (Both the Minikin and Skia parsers now
allow the font file name on a separate line in the fonts.xml file).

This information is then passed through to Skia in order to select the
desired variation. The Skia parser already parses this way and has for
some time, so Chrome and WebView can already read this format.

Change-Id: I15623fe864fa92b2bf0705af5e389daedfb77e5c
(cherry picked from commit b8e367fb)