Add comments for struct members that were changed from size_t to uin32_t or uint64_t for IPC safety.

media/cast/logging/logging_defines.h

Index: media/cast/logging/logging_defines.h
diff --git a/media/cast/logging/logging_defines.h b/media/cast/logging/logging_defines.h
index 7d17507367345aa0c91bb9fa9925b111947db3f1..33ea72dc81b518e27346a2a09ef120662525e245 100644
--- a/media/cast/logging/logging_defines.h
+++ b/media/cast/logging/logging_defines.h
@@ -64,6 +64,8 @@ struct FrameEvent {
   int height;
 
   // Size of encoded frame in bytes. Only set for FRAME_ENCODED event.
+  // Note: we use uint32_t instead of size_t for byte count because this struct
+  // is sent over IPC which could span 32 & 64 bit processes.
   uint32_t size;
 
   // Time of event logged.