Make GetCodingFrameNum const function

Change-Id: I6a5a2400cfb6e122c77667e0950c80026c48a1f6
diff --git a/vp9/simple_encode.cc b/vp9/simple_encode.cc
index 6bf55c9..f34df68 100644
--- a/vp9/simple_encode.cc
+++ b/vp9/simple_encode.cc
@@ -279,7 +279,7 @@
   encode_command_reset_external_quantize_index(&impl_ptr_->cpi->encode_command);
 }
 
-int SimpleEncode::GetCodingFrameNum() {
+int SimpleEncode::GetCodingFrameNum() const {
   assert(impl_ptr_->first_pass_stats.size() - 1 > 0);
   // These are the default settings for now.
   const int multi_layer_arf = 0;
diff --git a/vp9/simple_encode.h b/vp9/simple_encode.h
index 5d6972e..f2344a2 100644
--- a/vp9/simple_encode.h
+++ b/vp9/simple_encode.h
@@ -74,7 +74,7 @@
   // Gets the number of coding frames for the video. The coding frames include
   // show frame and no show frame.
   // This function should be called after ComputeFirstPassStats().
-  int GetCodingFrameNum();
+  int GetCodingFrameNum() const;
 
  private:
   class EncodeImpl;