[Fuchsia] Fix DCHECK in SysmemBufferWriterQueue::ReleaseBuffer()

Previously SysmemBufferWriterQueue wasn't handling properly the case
when a packet is destroyed after ResetBuffers(). Updated ResetBuffers()
to drop weak pointers, to ensure that ReleaseBuffer() is not called
after ResetBuffers().

Bug: 1021646
Change-Id: I75045ce7427eb0c539bd198bb73a5a742531f0f2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1899319
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Commit-Queue: Yuchen Liu <yucliu@chromium.org>
Auto-Submit: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Yuchen Liu <yucliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712730}

media/fuchsia/common/sysmem_buffer_writer_queue.cc

@@ -126,6 +126,10 @@ void SysmemBufferWriterQueue::ResetBuffers() {
   writer_.reset();
   send_packet_cb_ = SendPacketCB();
   end_of_stream_cb_ = EndOfStreamCB();
+
+  // Invalidate weak pointers, so ReleaseBuffer() is not called for the old
+  // buffers.
+  weak_factory_.InvalidateWeakPtrs();
 }
 
 void SysmemBufferWriterQueue::ResetPositionAndPause() {