Recently-added GENERATED_FILES support and support for specifying
per-file methods for GENERATED_FILES means that we can move the rules
for generating tab SVGs in browser/themes/ to moz.build. As a bonus, we
can also eliminate duplicate between the rules by moving them to a
shared mozbuild file in browser/themes/, rather than individual
browser/themes/{linux,osx,windows}/ Makefiles.
Nathan Froyd