[Platform] Increase default stack size on Mac OS X to 1MB.

The default stack size of a background thread is 512KB on MacOSX. We default to
1MB stack checks when compiling JS code, so we need to increase this limit
to enable compilation of JS code onto background threads.

Corresponding Chromium CL is https://codereview.chromium.org/2640803002/

BUG=v8:5203

Review-Url: https://codereview.chromium.org/2653673007
Cr-Commit-Position: refs/heads/master@{#42650}

src/base/platform/platform-posix.cc

@@ -620,12 +620,15 @@ void Thread::Start() {
   result = pthread_attr_init(&attr);
   DCHECK_EQ(0, result);
   size_t stack_size = stack_size_;
-#if V8_OS_AIX
   if (stack_size == 0) {
-    // Default on AIX is 96KB -- bump up to 2MB
+#if V8_OS_MACOSX
+    // Default on Mac OS X is 512kB -- bump up to 1MB
+    stack_size = 1 * 1024 * 1024;
+#elif V8_OS_AIX
+    // Default on AIX is 96kB -- bump up to 2MB
     stack_size = 2 * 1024 * 1024;
-  }
 #endif
+  }
   if (stack_size > 0) {
     result = pthread_attr_setstacksize(&attr, stack_size);
     DCHECK_EQ(0, result);