Increase default stack size from 8K to 12K.

For internal bug 1844105.
diff --git a/vm/Thread.h b/vm/Thread.h
index 86a7845..9c15f9e 100644
--- a/vm/Thread.h
+++ b/vm/Thread.h
@@ -72,7 +72,7 @@
 #define kInternalRefMax         4096    /* mainly a sanity check */
 
 #define kMinStackSize       (512 + STACK_OVERFLOW_RESERVE)
-#define kDefaultStackSize   (8*1024)    /* two 4K pages */
+#define kDefaultStackSize   (12*1024)   /* three 4K pages */
 #define kMaxStackSize       (256*1024 + STACK_OVERFLOW_RESERVE)
 
 /*