Avoid warnings in DownstreamCache tests about an uncleaned up rewrite_driver

by making sure that other_rewrite_driver gets cleaned up: the test sets
RewriteTestBase into managed mode, which basically means it does
rewrite_driver = NewRewriteDriver
other_rewrite_driver = NewRewriterDriver
... but then nothing actually does anything with other_rewrite_driver, making it
stick-around until factory shutdown does emergency cleanup.

(see https://github.com/pagespeed/mod_pagespeed/issues/1421)

net/instaweb/rewriter/public/rewrite_test_base.h

@@ -241,7 +241,9 @@ class RewriteTestBase : public RewriteOptionsTestBase {
   // Use managed rewrite drivers for the test so that we see the same behavior
   // in tests that we see in real servers. By default, tests use unmanaged
   // drivers so that _test.cc files can add options after the driver was created
-  // and before the filters are added.
+  // and before the filters are added.  Note that this will only clean them up
+  // via shutdown codepath if you don't actually use them, unless an explicit
+  // Cleanup() call is made.
   void SetUseManagedRewriteDrivers(bool use_managed_rewrite_drivers);
 
   GoogleString CssLinkHref(const StringPiece& url) {

net/instaweb/rewriter/rewrite_driver_test.cc

@@ -2146,11 +2146,19 @@ TEST_F(RewriteDriverTest, SetRequestHeadersPopulatesWebpNoAccept) {
 // rewritten in the very first go.
 class DownstreamCacheWithPossiblePurgeTest : public RewriteDriverTest {
  protected:
-  void SetUp() {
+  void SetUp() override {
     options()->EnableFilter(RewriteOptions::kExtendCacheCss);
     SetUseManagedRewriteDrivers(true);
     RewriteDriverTest::SetUp();
   }
+
+  void TearDown() override {
+    // We need to clean up the other rewrite driver manually since we don't
+    // parse anything through it --- NewRewriteDriver is called, but nothing
+    // else is done otherwise.
+    other_rewrite_driver()->Cleanup();
+    RewriteDriverTest::TearDown();
+  }
 };
 
 // This class has CollapseWhitespace filter enabled and has no possibility of
@@ -2158,11 +2166,19 @@ class DownstreamCacheWithPossiblePurgeTest : public RewriteDriverTest {
 // in the very first go.
 class DownstreamCacheWithNoPossiblePurgeTest : public RewriteDriverTest {
  protected:
-  void SetUp() {
+  void SetUp() override {
     options()->EnableFilter(RewriteOptions::kCollapseWhitespace);
     SetUseManagedRewriteDrivers(true);
     RewriteDriverTest::SetUp();
   }
+
+  void TearDown() override {
+    // We need to clean up the other rewrite driver manually since we don't
+    // parse anything through it --- NewRewriteDriver is called, but nothing
+    // else is done otherwise.
+    other_rewrite_driver()->Cleanup();
+    RewriteDriverTest::TearDown();
+  }
 };
 
 TEST_F(DownstreamCacheWithPossiblePurgeTest, DownstreamCacheEnabled) {