Bug 898525. Separate out Gfx helpers into GfxMessageUtils.h. r=bent

This helps us include less.

--HG--
extra : rebase_source : bb9ca934a9dade9ef21cecfee7ae8e860f9c60c1
No bug, Automated HSTS preload list update from host bld-centos6-hp-008 - a=hsts-update
No bug, Automated blocklist update from host bld-linux64-ix-027 - a=blocklist-update
Merge m-i to m-c
Merge m-c to m-i