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

chrome/common/spellcheck_marker.h

Index: chrome/common/spellcheck_marker.h
diff --git a/chrome/common/spellcheck_marker.h b/chrome/common/spellcheck_marker.h
index 4305c22f0efab67ff359e506f6e4e5f4f5a03a74..866ec03cb53cfb3212f811b7a355e446cf217e36 100644
--- a/chrome/common/spellcheck_marker.h
+++ b/chrome/common/spellcheck_marker.h
@@ -29,6 +29,9 @@ class SpellCheckMarker {
       : hash(hash), offset(offset) {}
 
   uint32_t hash;
+  // Note: we use uint32_t instead of size_t because this struct is sent over
+  // IPC which could span 32 & 64 bit processes. This is fine since the offset
+  // shouldn't exceed UINT32_MAX even on 64 bit builds.
   uint32_t offset;
 };