Bug 1239210 - Don't process WebIDL files when not compiling; r=glandium

The output of WebIDL codegen is a bunch of .h and .cpp files. These
aren't relevant when not compiling.

This change appears to shave ~3-4s off the "export" tier time for
--disable-compile-environment builds on my i7-6700K because WebIDL
codegen is currently a long pole in that tier. After this patch,
artifact clobber builds are ~43.5s.

--HG--
extra : rebase_source : 95c96e1631ddb8d2efc89d8462fe0f8ade1ecf1f

dom/bindings/Makefile.in

@@ -4,6 +4,8 @@
 
 webidl_base := $(topsrcdir)/dom/webidl
 
+ifdef COMPILE_ENVIRONMENT
+
 # Generated by moz.build
 include webidlsrcs.mk
 
@@ -54,6 +56,8 @@ codegen.pp: $(codegen_dependencies)
 compiletests:
 	$(call SUBMAKE,libs,test)
 
+endif
+
 GARBAGE += \
   codegen.pp \
   codegen.json \