'injected_files' should be base 64 encoded.

If an instance build needs to be rescheduled, send the original
base 64 encoded injected_files parameter.

bug 1161479

Change-Id: I4dad95e95cd27f7f128989daa36b3a2bbf132c73
(cherry picked from commit f6af4f753676088318b3944e9e41d698950093d9)