Issue #26 Part 2a: make more EME code conditional.

- Exclude missed MediaKey functions and CDMProxy code.
- Exclude EME APIs frm being built (webidl change)
- Fix tests in --disable-eme state

dom/media/platforms/moz.build

@@ -30,11 +30,13 @@ UNIFIED_SOURCES += [
 ]
 
 DIRS += [
-    'agnostic/eme',
     'agnostic/gmp',
     'omx'
 ]
 
+if CONFIG['MOZ_EME']:
+    DIRS += ['agnostic/eme']
+
 if CONFIG['MOZ_WMF']:
     DIRS += [ 'wmf' ];