Adding macro to enable changing SSL library (Part 1)

ios/chrome/browser/ios_chrome_io_thread.mm

Index: ios/chrome/browser/ios_chrome_io_thread.mm
diff --git a/ios/chrome/browser/ios_chrome_io_thread.mm b/ios/chrome/browser/ios_chrome_io_thread.mm
index ef492dc3b698b1c7b95aeead7c512743dbbc3229..31b5c5a3b6764e10a65935547e4133863181bfed 100644
--- a/ios/chrome/browser/ios_chrome_io_thread.mm
+++ b/ios/chrome/browser/ios_chrome_io_thread.mm
@@ -140,12 +140,18 @@ const char kNpnTrialDisabledGroupNamePrefix[] = "Disable";
 // Used for the "system" URLRequestContext.
 class SystemURLRequestContext : public net::URLRequestContext {
  public:
-  SystemURLRequestContext() { net::SetURLRequestContextForNSSHttpIO(this); }
+  SystemURLRequestContext() {
+#if defined(USE_NSS_VERIFIER)
+    net::SetURLRequestContextForNSSHttpIO(this);
+#endif
+  }
 
  private:
   ~SystemURLRequestContext() override {
     AssertNoURLRequests();
+#if defined(USE_NSS_VERIFIER)
     net::SetURLRequestContextForNSSHttpIO(nullptr);
+#endif
   }
 };
 
@@ -377,7 +383,9 @@ void IOSChromeIOThread::Init() {
   TRACE_EVENT0("startup", "IOSChromeIOThread::Init");
   DCHECK_CURRENTLY_ON(web::WebThread::IO);
 
+#if defined(USE_NSS_VERIFIER)
   net::SetMessageLoopForNSSHttpIO();
+#endif
 
   const base::CommandLine& command_line =
       *base::CommandLine::ForCurrentProcess();
@@ -474,7 +482,9 @@ void IOSChromeIOThread::Init() {
 }
 
 void IOSChromeIOThread::CleanUp() {
+#if defined(USE_NSS_VERIFIER)
   net::ShutdownNSSHttpIO();
+#endif
 
   system_url_request_context_getter_ = nullptr;