fix Arena total-size accounting across clear

Summary: Fix `Arena` total-size accounting across calls to `clear`, as reported by member `totalSize`.

Reviewed By: luciang

Differential Revision: D25656867

fbshipit-source-id: b5c7bb0e95199a9fa2c94c04de65b8f494eef146

folly/memory/Arena.h

@@ -200,6 +200,7 @@ class Arena {
   void freeLargeBlocks() {
     largeBlocks_.clear_and_dispose([this](LargeBlock* b) {
       auto size = b->allocSize;
+      totalAllocatedSize_ -= size;
       b->~LargeBlock();
       AllocTraits::deallocate(alloc(), reinterpret_cast<char*>(b), size);
     });

folly/memory/test/ArenaTest.cpp

@@ -216,7 +216,7 @@ TEST(Arena, Clear) {
     }
 
     EXPECT_EQ(arena.bytesUsed(), bytesUsed);
-    EXPECT_GT(arena.totalSize(), totalSize);
+    EXPECT_EQ(arena.totalSize(), totalSize);
 
     arena.clear();
   }