Fixed access to `constraint_flags` array in ParseHEVCCodecId function.

This patch adds necessary offset when accessing this array.
`constraint_flags` is an array of 6 elements, however in
`for` loop it can be accessed with indices from 4 to 9
instead of indices from 0 to 5.

Review-Url: https://codereview.chromium.org/2320063002
Cr-Commit-Position: refs/heads/master@{#417351}

media/base/video_codecs.cc

@@ -278,7 +278,7 @@ bool ParseHEVCCodecId(const std::string& codec_id,
       DVLOG(4) << __func__ << ": invalid constraint byte=" << elem[i];
       return false;
     }
-    constraint_flags[i] = constr_byte;
+    constraint_flags[i - 4] = constr_byte;
   }
 
   return true;