Fix some leaks and failures under valgrind in JS extensions unit tests.

This change:
- Adds running the GC until the v8 heap size reaches steady state during
  ModuleSystemTest::TearDown. This is necessary to ensure that
  gin::Wrappable objects are freed, as finalizers for JS objects aren't
  run during isolate shut down.
- Removes a racy check that failed under valgrind.

BUG=389016,406487

Review URL: https://codereview.chromium.org/502893002

Cr-Commit-Position: refs/heads/master@{#294779}

extensions/renderer/api/serial/serial_api_unittest.cc

@@ -409,12 +409,6 @@ class SerialApiTest : public ApiTestBase {
         &SerialApiTest::CreateSerialService, base::Unretained(this)));
   }
 
-  virtual void TearDown() OVERRIDE {
-    if (io_handler_.get())
-      EXPECT_TRUE(io_handler_->HasOneRef());
-    ApiTestBase::TearDown();
-  }
-
   scoped_refptr<TestIoHandlerBase> io_handler_;
 
  private:

extensions/renderer/module_system_test.cc

@@ -211,18 +211,33 @@ v8::Handle<v8::Object> ModuleSystemTestEnvironment::CreateGlobal(
 
 ModuleSystemTest::ModuleSystemTest()
     : isolate_(v8::Isolate::GetCurrent()),
-      env_(CreateEnvironment()),
       should_assertions_be_made_(true) {
 }
 
 ModuleSystemTest::~ModuleSystemTest() {
 }
 
+void ModuleSystemTest::SetUp() {
+  env_ = CreateEnvironment();
+}
+
 void ModuleSystemTest::TearDown() {
   // All tests must assert at least once unless otherwise specified.
   EXPECT_EQ(should_assertions_be_made_,
             env_->assert_natives()->assertion_made());
   EXPECT_FALSE(env_->assert_natives()->failed());
+  env_.reset();
+  v8::HeapStatistics stats;
+  isolate_->GetHeapStatistics(&stats);
+  size_t old_heap_size = 0;
+  // Run the GC until the heap size reaches a steady state to ensure that
+  // all the garbage is collected.
+  while (stats.used_heap_size() != old_heap_size) {
+    old_heap_size = stats.used_heap_size();
+    isolate_->RequestGarbageCollectionForTesting(
+        v8::Isolate::kFullGarbageCollection);
+    isolate_->GetHeapStatistics(&stats);
+  }
 }
 
 scoped_ptr<ModuleSystemTestEnvironment> ModuleSystemTest::CreateEnvironment() {

extensions/renderer/module_system_test.h

@@ -81,6 +81,7 @@ class ModuleSystemTest : public testing::Test {
   ModuleSystemTest();
   virtual ~ModuleSystemTest();
 
+  virtual void SetUp() OVERRIDE;
   virtual void TearDown() OVERRIDE;
 
  protected: