Make ScopedTaskEnvironment the first member of QuicHttpProxyBackendStreamTest.

The documentation for ScopedTaskEnvironment recommends making it the
first member of the test fixture:
  https://cs.chromium.org/chromium/src/base/test/scoped_task_environment.h?l=54-59&rcl=376db7a96e0287f0d010acd18e7a379d190cac71

We are chaning this particular case because we want to make the
constructor of ScopedTaskEnvironment set a global variable that is
read by PlatformThread::SetCurrentThreadPriority(). If
ScopedTaskEnvironment is initialized late in the test execution,
setting this global variable races with a call to
PlatfromThread::SetCurrentThreadPriority() on the thread created
from this stack:
  base::(anonymous namespace)::CreateThread
  base::PlatformThread::CreateWithPriority
  base::Thread::StartWithOptions
  net::test::QuicHttpProxyBackendStreamTest::CreateTestBackendProxy
  net::test::QuicHttpProxyBackendStreamTest::SetUp

Bug: 902441
Change-Id: I4869410f43383bd7f94f68fc7790e3305f3d1b4a
Reviewed-on: https://chromium-review.googlesource.com/c/1320654
Commit-Queue: François Doray <fdoray@chromium.org>
Reviewed-by: Ryan Hamilton <rch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606259}

net/tools/quic/quic_http_proxy_backend_stream_test.cc

@@ -136,7 +136,6 @@ class TestQuicServerStreamDelegate
   bool send_success_;
   bool did_complete_;
   std::unique_ptr<QuicHttpProxyBackendStream> quic_backend_stream_;
-  base::test::ScopedTaskEnvironment scoped_task_environment;
   const scoped_refptr<base::SingleThreadTaskRunner> task_runner_ =
       base::ThreadTaskRunnerHandle::Get();
   base::RunLoop run_loop_;
@@ -185,6 +184,7 @@ class QuicHttpProxyBackendStreamTest : public QuicTest {
   }
 
  protected:
+  base::test::ScopedTaskEnvironment scoped_task_environment_;
   std::string backend_url_;
   std::unique_ptr<QuicHttpProxyBackend> proxy_backend_;
   std::unique_ptr<QuicHttpProxyBackend> proxy_backend_fail_;
@@ -479,4 +479,4 @@ TEST_F(QuicHttpProxyBackendStreamTest, SendRequestToBackendHopHeaders) {
 }
 
 }  // namespace test
-}  // namespace net
\ No newline at end of file
+}  // namespace net