Enable asynchronous glReadPixels on Windows.

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 8e222e135bf6a329988141138a657e87f8be6c79..578870d2b045f03a317664eb3ba3d5ae5e0089aa 100644
--- a/gpu/command_buffer/client/fenced_allocator.h
+++ b/gpu/command_buffer/client/fenced_allocator.h
@@ -35,6 +35,9 @@ class GPU_EXPORT FencedAllocator {
   // Invalid offset, returned by Alloc in case of failure.
   static const Offset kInvalidOffset = 0xffffffffU;
 
+  // Allocation alignment, must be a power of two.
+  static const unsigned int kAllocAlignment = 16;
+
   // Creates a FencedAllocator. Note that the size of the buffer is passed, but
   // not its base address: everything is handled as offsets into the buffer.
   FencedAllocator(unsigned int size,