[heap] Fix MemoryAllocator::AllocateAlignedMemory.

AllocateAlignedMemory calls ReserveAlignedMemory which increments size_
value. In some cases AllocateAlignedMemory can return NULL value. Before
returning NULL, AllocateAlignedMemory should decrease size_ value.

TEST=cctest/test-spaces/LargeObjectSpace
BUG=

Review-Url: https://codereview.chromium.org/2418733002
Cr-Commit-Position: refs/heads/master@{#40262}

src/heap/spaces.cc

@@ -478,6 +478,7 @@ Address MemoryAllocator::AllocateAlignedMemory(
     // Failed to commit the body. Release the mapping and any partially
     // commited regions inside it.
     reservation.Release();
+    size_.Decrement(reserve_size);
     return NULL;
   }