Make FencedAlloctor fail on size = 0.

gpu/command_buffer/client/fenced_allocator.h

Index: gpu/command_buffer/client/fenced_allocator.h
diff --git a/gpu/command_buffer/client/fenced_allocator.h b/gpu/command_buffer/client/fenced_allocator.h
index 3ccad45951e2fd0b45a94d49e88239c47941ae6d..486063b5c06e35ab30ceaa529618ce8c4e1dc81d 100644
--- a/gpu/command_buffer/client/fenced_allocator.h
+++ b/gpu/command_buffer/client/fenced_allocator.h
@@ -132,8 +132,13 @@ class GPU_EXPORT FencedAllocator {
   // the other functions that return a block index).
   Offset AllocInBlock(BlockIndex index, unsigned int size);
 
+  // Frees allocations of size = 0.
+  void FreeZeroAlloc(FencedAllocator::Offset offset);
+
   CommandBufferHelper *helper_;
   Container blocks_;
+  Offset zero_offset_;
+  std::vector<bool> zero_allocs_;
 
   DISALLOW_IMPLICIT_CONSTRUCTORS(FencedAllocator);
 };