Do not regenerate JNI files (and subsequently the .so file) if they have
not changed. This will happen if you edit a java file that has a native
counterpart, but the native signatures were not touched.
We do not enable this all the time as some build systems require that
when you modify an input, the output should also be modified. This
also will result in more command line output, so we'll keep it behind
a developer flag for now.
BUG=
Review URL: https://chromiumcodereview.appspot.com/12314025
git-svn-id: http://src.chromium.org/svn/trunk/src/build@183936 4ff67af0-8c30-449e-8e8b-ad334ec8d88c