[debugger] add comment on bytecode operand scaling.

TBR=mstarzinger@chromium.org

Change-Id: I13447a31518226d0d86365b0c41d40ac2f688275
Reviewed-on: https://chromium-review.googlesource.com/722702
Commit-Queue: Yang Guo <yangguo@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49128}

src/runtime/runtime-debug.cc

@@ -55,6 +55,9 @@ RUNTIME_FUNCTION_RETURN_PAIR(Runtime_DebugBreakOnBytecode) {
     // sees the return bytecode rather than the DebugBreak.
     interpreted_frame->PatchBytecodeArray(bytecode_array);
   }
+  // We do not have to deal with operand scale here. If the bytecode at the
+  // break is prefixed by operand scaling, we would have patched over the
+  // scaling prefix. We now simply dispatch to the handler for the prefix.
   return MakePair(isolate->debug()->return_value(),
                   isolate->interpreter()->GetBytecodeHandler(
                       bytecode, interpreter::OperandScale::kSingle));