Avoid integer overflow when computing the size of new space in bytes.

runtime/platform/globals.h

Index: runtime/platform/globals.h
diff --git a/runtime/platform/globals.h b/runtime/platform/globals.h
index b86a23df163239463b909e9bf7ef3fad924a55bc..95585b15e2ba13cee9221582260782d646f47193 100644
--- a/runtime/platform/globals.h
+++ b/runtime/platform/globals.h
@@ -167,9 +167,9 @@ const int kBitsPerByteLog2 = 3;
 const int kBitsPerWord = kWordSize * kBitsPerByte;
 
 // System-wide named constants.
-const int KB = 1024;
-const int MB = KB * KB;
-const int GB = KB * KB * KB;
+const intptr_t KB = 1024;
+const intptr_t MB = KB * KB;
+const intptr_t GB = KB * KB * KB;
 const intptr_t kIntptrOne = 1;
 const intptr_t kIntptrMin = (kIntptrOne << (kBitsPerWord - 1));
 const intptr_t kIntptrMax = ~kIntptrMin;