Better comment for a bailout case in the SafeStackFrameIterator.

We should explain why we default to StackFrame::OPTIMIZED rather
than StackFrame::INTERPRETED when we don't have a great idea
about what kind of frame we are looking at.

Bug: 
Change-Id: I9d3cc84def6164ef79a4792822d52c11dfe8a448
Reviewed-on: https://chromium-review.googlesource.com/668450Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Michael Stanton <mvstanton@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48565}

src/frames.cc

@@ -238,6 +238,11 @@ SafeStackFrameIterator::SafeStackFrameIterator(
       }
     } else {
       // Mark the frame as OPTIMIZED if we cannot determine its type.
+      // We chose OPTIMIZED rather than INTERPRETED because it's closer to
+      // the original value of StackFrame::JAVA_SCRIPT here, in that JAVA_SCRIPT
+      // referred to full-codegen frames (now removed from the tree), and
+      // OPTIMIZED refers to turbofan frames, both of which are generated
+      // code. INTERPRETED frames refer to bytecode.
       // The frame anyways will be skipped.
       type = StackFrame::OPTIMIZED;
       // Top frame is incomplete so we cannot reliably determine its type.