IDL compiler: fix order of constants in template

Constants were in the wrong place in the interface.cpp template;
this was revealed when I merged test IDL files in:
IDL compiler: Consolidate test IDL files
https://codereview.chromium.org/101403002/
...and this CL fixes it.

BUG=239771
NOTRY=true
TBR=haraken

Review URL: https://codereview.chromium.org/103553002

git-svn-id: svn://svn.chromium.org/blink/trunk@163139 bbb929c8-8fbe-4397-9dbb-9b2b20218538