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

extensions/common/stack_frame.h

Index: extensions/common/stack_frame.h
diff --git a/extensions/common/stack_frame.h b/extensions/common/stack_frame.h
index b174ffc71ea47240321e4900b694adac774f0ec6..033784a285dee23b1a89aad9ebeb97f930a2f802 100644
--- a/extensions/common/stack_frame.h
+++ b/extensions/common/stack_frame.h
@@ -29,6 +29,9 @@ struct StackFrame {
 
   bool operator==(const StackFrame& rhs) const;
 
+  // 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 line numbers
+  // and column numbers shouldn't exceed UINT32_MAX even on 64 bit builds.
   uint32_t line_number;
   uint32_t column_number;
   base::string16 source;