Bug 1068846 - Enable generational GC for SeaMonkey plus some minor updates to confvars.sh r=KaiRo a=bustage fix on CLOSED TREE

Bug 619558 - Enable generational GC on desktop
Bug 753203 - Enable exact rooting by default in all configurations
+JSGC_GENERATIONAL=1
Bug 881997 Use confvars to establish webgl conformance
+MOZ_WEBGL_CONFORMANT=1
Bug 891705 [MediaEncoder] Implement WebM 1.0 container writer.
+MOZ_WEBM_ENCODER=1
Bug 763180 - NS_XRE_ENABLE_EXTENSION_MANAGER flag does nothing - remove it
-MOZ_EXTENSION_MANAGER=1