[coverage] Correctly free DebugInfo in the absence of breakpoints

It's quite possible for DebugInfos to exist without the presence of a
bytecode array, since DebugInfos are created for all functions for which
we have a CoverageInfo. Free such objects properly.

Also move the corresponding deletion of CoverageInfos on unload up
before the early exit.

Bug: v8:6000
Change-Id: Idde45b222290aa8b6828b61ff2251918b8ed2aed
Reviewed-on: https://chromium-review.googlesource.com/664811
Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48024}

src/debug/debug.cc

@@ -337,13 +337,12 @@ bool Debug::Load() {
 void Debug::Unload() {
   ClearAllBreakPoints();
   ClearStepping();
+  if (FLAG_block_coverage) RemoveAllCoverageInfos();
   RemoveDebugDelegate();
 
   // Return debugger is not loaded.
   if (!is_loaded()) return;
 
-  if (FLAG_block_coverage) RemoveAllCoverageInfos();
-
   // Clear debugger context global handle.
   GlobalHandles::Destroy(Handle<Object>::cast(debug_context_).location());
   debug_context_ = Handle<Context>();
@@ -642,8 +641,11 @@ void Debug::ApplyBreakPoints(Handle<DebugInfo> debug_info) {
 }
 
 void Debug::ClearBreakPoints(Handle<DebugInfo> debug_info) {
+  // If we attempt to clear breakpoints but none exist, simply return. This can
+  // happen e.g. CoverageInfos exit but no breakpoints are set.
+  if (!debug_info->HasDebugBytecodeArray()) return;
+
   DisallowHeapAllocation no_gc;
-  DCHECK(debug_info->HasDebugBytecodeArray());
   for (BreakIterator it(debug_info); !it.Done(); it.Next()) {
     it.ClearDebugBreak();
   }