I420VideoSource: normalize framerate types

ctor inputs are ints as are vpx_rational_t members

Change-Id: I62a39bf3df123727a872e40b74e3ee9e55ef2ede

test/i420_video_source.h

@@ -108,8 +108,8 @@ class I420VideoSource : public VideoSource {
   unsigned int frame_;
   unsigned int width_;
   unsigned int height_;
-  unsigned int framerate_numerator_;
-  unsigned int framerate_denominator_;
+  int framerate_numerator_;
+  int framerate_denominator_;
 };
 
 }  // namespace libvpx_test