Bug 950370 - Capture WebIDL example interfaces in moz.build; r=glandium

Previously, the set of WebIDL example interfaces to build was hardcoded
in the mozwebidlcodegen Python package. Unfortunately, the example
interfaces in that set were dependent on test-only bindings files,
resulting in build failures when tests were disabled (because those
test-only bindings were defined in a test directory).

In this patch, we now declare example interfaces in moz.build files
so the set of example interfaces lives next to the set of defined
bindings files. The example interfaces are defined in a test moz.build
file - in the same file declaring the bindings files that contain the
interfaces - so the example interfaces only get picked up if the
corresponding bindings are defined.

We could probably switch WebIDL moz.build variables to use lists with
flags. That would be good followup fodder. For now, let's fix the build.

--HG--
extra : rebase_source : 00070a6d560625bc84ab9b10d6848ea037f8f5ed
extra : amend_source : 0890ecef444ba58533fcfaa333773d78deafd7f3