Fix regen of EventInterfaces.in by depending on list

Currently EventInterfaces.in isn't regenerated correctly if the *list*
of files changes -- it only rebuilds if one of the *files* is modified,
but not if a file is deleted, for instance.

This is fixed by explicitly depending on the list of files, which should
be done anyway (it's an argument to the command).

Documented at:
https://code.google.com/p/gyp/wiki/GypUserDocumentation

TBR=Nico
BUG=368875

Review URL: https://codereview.chromium.org/265803002

git-svn-id: svn://svn.chromium.org/blink/trunk@173054 bbb929c8-8fbe-4397-9dbb-9b2b20218538