Bug 979665 - Properly detect WebIDL type changes; r=froydnj

This patch fixes a bug in WebIDL code generation that could be triggered
if an input file changed locations but the original file remained on
disk. When computing the set of source .webidl files to perform code
generation on, we failed to prune files that were no longer part of the
active set of input files. References to files in old locations would
get pulled in to the regeneration set, leading to a cryptic KeyError
during code generation.

--HG--
extra : rebase_source : 23f1b973133960cfa954d44b20175fc7a369169a