Removing dead TransportDIB member sequence_num_

Nobody set or read the ui::surface::TransportDIB member
sequence_num_ and that made the compiler upset in jumbo builds
where the compiler sees more of the whole picture.

Change-Id: Ia59e824e9e6175870065336c6403958878d2dba0
Reviewed-on: https://chromium-review.googlesource.com/826063Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#524031}

ui/surface/transport_dib.h

@@ -81,7 +81,6 @@ class SURFACE_EXPORT TransportDIB {
 
   explicit TransportDIB(base::SharedMemoryHandle dib);
   base::SharedMemory shared_memory_;
-  uint32_t sequence_num_;
   size_t size_;  // length, in bytes
 
   DISALLOW_COPY_AND_ASSIGN(TransportDIB);