InvokeCallbackTask should use UniquePersistent to hold callback.

Before this patch, InvokeCallbackTask was holding onto a callback using v8::Persistent, and v8::Persistent do not release on d-tor by default.
This patch changes it to use UniquePersistent, so it will release callback on |~InvokeCallbackTask|.

BUG=None

Review URL: https://codereview.chromium.org/232733002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@262975 0039d316-1c4b-4281-b951-d872f2087c98

content/shell/renderer/test_runner/test_runner.cc

@@ -104,7 +104,7 @@ class InvokeCallbackTask : public WebMethodTask<content::TestRunner> {
   }
 
  private:
-  v8::Persistent<v8::Function> callback_;
+  v8::UniquePersistent<v8::Function> callback_;
 };
 
 class TestRunnerBindings : public gin::Wrappable<TestRunnerBindings> {