Use TaskEnvironment in V8VarConverterTest

Bug: 1006787
Change-Id: I1882b04323570828ac9dcea7a72dcc2c9843b5fa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1819256Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Tomasz Moniuszko <tmoniuszko@opera.com>
Cr-Commit-Position: refs/heads/master@{#698890}

content/renderer/pepper/v8_var_converter_unittest.cc

@@ -239,8 +239,8 @@ class V8VarConverterTest : public testing::Test {
   std::unique_ptr<V8VarConverter> converter_;
 
  private:
-  base::test::SingleThreadTaskEnvironment
-      task_environment_;  // Required to receive callbacks.
+  // Required to receive callbacks.
+  base::test::TaskEnvironment task_environment_;
 
   TestGlobals globals_;
 };