Add ExternalStringResourceBase::IsCompressible

src/objects.cc

Index: src/objects.cc
diff --git a/src/objects.cc b/src/objects.cc
index 732477cd81ff9852091de87e7a262a58e2bcecfc..cfea67b071ac479d713bb60ed3b0a3365e9b502c 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -1662,6 +1662,7 @@ bool String::MakeExternal(v8::String::ExternalStringResource* resource) {
   // Externalizing twice leaks the external resource, so it's
   // prohibited by the API.
   DCHECK(!this->IsExternalString());
+  DCHECK(!resource->IsCompressible());
 #ifdef ENABLE_SLOW_DCHECKS
   if (FLAG_enable_slow_asserts) {
     // Assert that the resource and the string are equivalent.
@@ -1724,6 +1725,7 @@ bool String::MakeExternal(v8::String::ExternalOneByteStringResource* resource) {
   // Externalizing twice leaks the external resource, so it's
   // prohibited by the API.
   DCHECK(!this->IsExternalString());
+  DCHECK(!resource->IsCompressible());
 #ifdef ENABLE_SLOW_DCHECKS
   if (FLAG_enable_slow_asserts) {
     // Assert that the resource and the string are equivalent.