[coverage] Handle repeated CoverageInfo deletions

Debug::ClearCoverageInfo may be called multiple times, for example
whenever coverage mode is switched to BestEffort. When encountering a
DebugInfo that does not have a CoverageInfo attached, simply do nothing
instead of failing a DCHECK.

TBR=yangguo@chromium.org

Bug: v8:6000
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I3910b21ab40e7db02af5f3b6add3ec3f86e5bd91
Reviewed-on: https://chromium-review.googlesource.com/588788
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46925}

src/objects/debug-objects.cc

@@ -171,15 +171,15 @@ bool DebugInfo::HasCoverageInfo() const {
 
 bool DebugInfo::ClearCoverageInfo() {
   DCHECK(FLAG_block_coverage);
-  DCHECK(HasCoverageInfo());
-  Isolate* isolate = GetIsolate();
-
-  set_coverage_info(isolate->heap()->undefined_value());
+  if (HasCoverageInfo()) {
+    Isolate* isolate = GetIsolate();
 
-  int new_flags = flags() & ~kHasCoverageInfo;
-  set_flags(new_flags);
+    set_coverage_info(isolate->heap()->undefined_value());
 
-  return new_flags == kNone;
+    int new_flags = flags() & ~kHasCoverageInfo;
+    set_flags(new_flags);
+  }
+  return flags() == kNone;
 }
 
 // Remove the specified break point object.