Bug 1027402 - Part 1: Remove set* from BaseProxyHandler. (r=bholley)
Bug 1027402 - Part 2: Mark Proxy Handler instances as const. (r=bholley)
Bug 1027402 - Part 3: Mark all Proxy Handler methods const. (r=jorendorff, r=bz)
Bug 1027402 - Part 4: Mark Proxy Handler uses const. (r=jorendorff, r=bz)