Fix --max_old_space_size=4096 integer overflow.

BUG=v8:3857
LOG=N

Review URL: https://codereview.chromium.org/897543002

Cr-Commit-Position: refs/heads/master@{#26510}

src/heap/heap.cc

@@ -5133,10 +5133,10 @@ bool Heap::ConfigureHeap(int max_semi_space_size, int max_old_space_size,
     max_semi_space_size_ = max_semi_space_size * MB;
   }
   if (max_old_space_size > 0) {
-    max_old_generation_size_ = max_old_space_size * MB;
+    max_old_generation_size_ = static_cast<intptr_t>(max_old_space_size) * MB;
   }
   if (max_executable_size > 0) {
-    max_executable_size_ = max_executable_size * MB;
+    max_executable_size_ = static_cast<intptr_t>(max_executable_size) * MB;
   }
 
   // If max space size flags are specified overwrite the configuration.
@@ -5144,10 +5144,11 @@ bool Heap::ConfigureHeap(int max_semi_space_size, int max_old_space_size,
     max_semi_space_size_ = FLAG_max_semi_space_size * MB;
   }
   if (FLAG_max_old_space_size > 0) {
-    max_old_generation_size_ = FLAG_max_old_space_size * MB;
+    max_old_generation_size_ =
+        static_cast<intptr_t>(FLAG_max_old_space_size) * MB;
   }
   if (FLAG_max_executable_size > 0) {
-    max_executable_size_ = FLAG_max_executable_size * MB;
+    max_executable_size_ = static_cast<intptr_t>(FLAG_max_executable_size) * MB;
   }
 
   if (FLAG_stress_compaction) {