Bug 1525199 - Part 2 - removed size_t from GfxVarUpdate IPC messages; r=jrmuizel

These are indexes into an array of prefs so we're nowhere near needing to worry
about >32-bit values.

Differential Revision: https://phabricator.services.mozilla.com/D19193

--HG--
extra : moz-landing-system : lando

gfx/ipc/GraphicsMessages.ipdlh

@@ -98,7 +98,7 @@ union GfxVarValue
 
 struct GfxVarUpdate
 {
-  size_t index;
+  uint32_t index;
   GfxVarValue value;
 };