Attach service to workspace disposal token.

src/VisualStudio/Core/Def/SymbolSearch/VisualStudioSymbolSearchService.cs

@@ -45,8 +45,6 @@ internal partial class VisualStudioSymbolSearchService : AbstractDelayStartedSer
         private readonly string _localSettingsDirectory;
         private readonly LogService _logService;
 
-        private readonly CancellationTokenSource _cancellationTokenSource = new CancellationTokenSource();
-
         [ImportingConstructor]
         [Obsolete(MefConstruction.ImportingConstructorMessage, error: true)]
         public VisualStudioSymbolSearchService(
@@ -93,7 +91,7 @@ private async Task<ISymbolSearchUpdateEngine> GetEngineAsync(CancellationToken c
 
         private async Task UpdateSourceInBackgroundAsync(string sourceName)
         {
-            var engine = await GetEngineAsync(_cancellationTokenSource.Token).ConfigureAwait(false);
+            var engine = await GetEngineAsync(this.ThreadingContext.DisposalToken).ConfigureAwait(false);
             await engine.UpdateContinuouslyAsync(sourceName, _localSettingsDirectory).ConfigureAwait(false);
         }