Revert of Get rid of UnsafeCurrent in Sampler

src/isolate.h

Index: src/isolate.h
diff --git a/src/isolate.h b/src/isolate.h
index 74122db7cc333826363f78269ee76d4dad42b267..015f861301b1b05e48b8efbc5955a236147a17fe 100644
--- a/src/isolate.h
+++ b/src/isolate.h
@@ -493,6 +493,14 @@
         base::Thread::GetExistingThreadLocal(isolate_key_));
     DCHECK(isolate != NULL);
     return isolate;
+  }
+
+  // Like Current, but skips the check that |isolate_key_| was initialized.
+  // Callers have to ensure that themselves.
+  // DO NOT USE. The only remaining callsite will be deleted soon.
+  INLINE(static Isolate* UnsafeCurrent()) {
+    return reinterpret_cast<Isolate*>(
+        base::Thread::GetThreadLocal(isolate_key_));
   }
 
   // Usually called by Init(), but can be called early e.g. to allow