[debugger] remove mirror cache and v8::Debug::GetMirror.

test/cctest/test-debug.cc

Index: test/cctest/test-debug.cc
diff --git a/test/cctest/test-debug.cc b/test/cctest/test-debug.cc
index e053a4e604d71be5c10799679970c3e85fabe7ea..ce069884cb699499a00c337d311d1a125284fb71 100644
--- a/test/cctest/test-debug.cc
+++ b/test/cctest/test-debug.cc
@@ -4422,7 +4422,7 @@ TEST(HiddenPrototypePropertyMirror) {
             .FromJust());
 
   // The prototype (__proto__) for o0 should be o3 as o1 and o2 are hidden.
-  CHECK(CompileRun("o0_mirror.protoObject() == o3_mirror")
+  CHECK(CompileRun("o0_mirror.protoObject().value() == o3_mirror.value()")
             ->BooleanValue(context)
             .FromJust());
 }