Add a sanity check for detecting incorrect PreRunTestOnMainThread overloading

This CL is adding a sanity dcheck to avoid tests crashing on MAC only when
a chrome developer forgot to call the base function
"ContentBrowserTest::PreRunTestOnMainThread".


  class MyBrowserTest : public ContentBrowserTest {

   public:

    void PreRunTestOnMainThread() override {

      [...]

      ContentBrowserTest::PreRunTestOnMainThread();
  // Required, otherwise crash on MAC.
    }

I recently did that silly mistake and needed time to set up a MAC build to
debug it. I believe this test is cheap enough and worth saving developers time.

R=sky@chromium.org

Change-Id: Iec0ed2e19b216a35c675016ad6ff88dc9e1c86ad
Reviewed-on: https://chromium-review.googlesource.com/c/1407110Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Etienne Bergeron <etienneb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#626186}

content/public/test/content_browser_test.cc

@@ -140,9 +140,16 @@ void ContentBrowserTest::PreRunTestOnMainThread() {
 #if defined(OS_MACOSX)
   pool_->Recycle();
 #endif
+
+  pre_run_test_executed_ = true;
 }
 
 void ContentBrowserTest::PostRunTestOnMainThread() {
+  // This code is failing when the test is overriding PreRunTestOnMainThread()
+  // without the required call to ContentBrowserTest::PreRunTestOnMainThread().
+  // This is a common error causing a crash on MAC.
+  DCHECK(pre_run_test_executed_);
+
 #if defined(OS_MACOSX)
   pool_->Recycle();
 #endif

content/public/test/content_browser_test.h

@@ -48,7 +48,7 @@ class ContentBrowserTest : public BrowserTestBase {
   base::FilePath GetTestDataFilePath();
 
  private:
-  Shell* shell_;
+  Shell* shell_ = nullptr;
 
 #if defined(OS_MACOSX)
   // On Mac, without the following autorelease pool, code which is directly
@@ -66,6 +66,10 @@ class ContentBrowserTest : public BrowserTestBase {
   // ContentMain. For Android we set things up manually.
   std::unique_ptr<ShellMainDelegate> shell_main_delegate_;
 #endif
+
+  // Used to detect incorrect overriding of PreRunTestOnMainThread() with
+  // missung call to base implementation.
+  bool pre_run_test_executed_ = false;
 };
 
 }  // namespace content