[Fuchsia] Close BufferCollection on SysmemBufferPool destruction

Previously SysmemBufferPool wasn't calling Close(), which means that
sysmem would fail the collection after SysmemBufferPool is destroyed,
which prevent other clients from using that collection. Now the
destructor calls Close() to avoid this issue.

Change-Id: I0edd5cf9c79d9c0a14048211cebab215d76c953c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1820102
Auto-Submit: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Yuchen Liu <yucliu@chromium.org>
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#699900}

media/fuchsia/common/sysmem_buffer_pool.cc

@@ -63,7 +63,10 @@ SysmemBufferPool::SysmemBufferPool(
   });
 }
 
-SysmemBufferPool::~SysmemBufferPool() = default;
+SysmemBufferPool::~SysmemBufferPool() {
+  if (collection_)
+    collection_->Close();
+}
 
 fuchsia::sysmem::BufferCollectionTokenPtr SysmemBufferPool::TakeToken() {
   DCHECK(!shared_tokens_.empty());