[debugger] add precise mode for code coverage.

src/isolate.cc

Index: src/isolate.cc
diff --git a/src/isolate.cc b/src/isolate.cc
index d54654f9c6f607c30b9d0b73718c0bf4e5e7ead8..f80cb3bf9519d97a5a97299c6fd9a675f7f52cf8 100644
--- a/src/isolate.cc
+++ b/src/isolate.cc
@@ -2901,10 +2901,9 @@ Map* Isolate::get_initial_js_array_map(ElementsKind kind) {
   return nullptr;
 }
 
-
-bool Isolate::use_crankshaft() const {
+bool Isolate::use_crankshaft() {
   return FLAG_opt && FLAG_crankshaft && !serializer_enabled_ &&
-         CpuFeatures::SupportsCrankshaft();
+         CpuFeatures::SupportsCrankshaft() && !IsCodeCoverageEnabled();
 }
 
 bool Isolate::NeedsSourcePositionsForProfiling() const {
@@ -2913,6 +2912,15 @@ bool Isolate::NeedsSourcePositionsForProfiling() const {
          debug_->is_active() || logger_->is_logging();
 }
 
+bool Isolate::IsCodeCoverageEnabled() {

[jgruber, 2017/02/09 16:08:48]

Can we mark this const and keep constness above for use_crankshaft? 
And if it's performance-sensitive, perhaps move to the header for inlining.

[Yang, 2017/02/09 17:42:15]

I don't think this is performance sensitive. And unfortunately accessing the
heap is not marked as const. If I did that for the root list accessors, I'll
open a whole new can of worms that I'd rather not.

+  return heap()->code_coverage_list()->IsFixedArray();

[jgruber, 2017/02/09 16:08:48]

IsArrayList may be better here and elsewhere for clarity.

[Yang, 2017/02/09 17:42:15]

Done.

+}
+
+void Isolate::SetCodeCoverageList(Object* value) {
+  DCHECK(value->IsUndefined(this) || value->IsFixedArray());
+  heap()->set_code_coverage_list(value);
+}
+
 bool Isolate::IsArrayOrObjectPrototype(Object* object) {
   Object* context = heap()->native_contexts_list();
   while (!context->IsUndefined(this)) {