Prevent clang from optimizing away a malloc in a test.

base/security_unittest.cc

Index: base/security_unittest.cc
diff --git a/base/security_unittest.cc b/base/security_unittest.cc
index a7e3bc4bd6e72e95097fa198702337a7664fc7ac..f4c60a75db7291df8f4d51b8a495d48e6915a34d 100644
--- a/base/security_unittest.cc
+++ b/base/security_unittest.cc
@@ -42,7 +42,8 @@ int OnNoMemory(size_t) {
 
 void ExhaustMemoryWithMalloc() {
   for (;;) {
-    void* buf = malloc(kLargePermittedAllocation);
+    // Without the |volatile|, clang optimizes away the allocation.
+    void* volatile buf = malloc(kLargePermittedAllocation);
     if (!buf)
       break;
   }