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We don't have any jarjar rules yet, but add the empty file now in order
that the downstream build can enable jarjar. This way, when rules are
added later it will just work, instead of breaking the downstream build
temporarily.

Also, make sure that when a jarjar rule file is being used, it's
considered as an input to the JNI generator step, to make sure the JNI
header files get regenerated when the rules change.

BUG=

Review URL: https://chromiumcodereview.appspot.com/15888011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@203141 0039d316-1c4b-4281-b951-d872f2087c98