[api] unschedule deprecation for v8::Object::GetIsolate.

R=adamk@chromium.org

Bug: v8:7288
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: If8e2416a0e9b0915572c2ad37a45e3ea8ca3a75a
Reviewed-on: https://chromium-review.googlesource.com/952924Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51793}

include/v8.h

@@ -3520,7 +3520,7 @@ class V8_EXPORT Object : public Value {
   /**
    * Return the isolate to which the Object belongs to.
    */
-  V8_DEPRECATE_SOON("Keep track of isolate correctly", Isolate* GetIsolate());
+  Isolate* GetIsolate();
 
   static Local<Object> New(Isolate* isolate);