Add check for |destination_node_| before querying info from context

The refactoring CL
(https://chromium-review.googlesource.com/c/chromium/src/+/1115516)
removed the redundant checks for the destination node in some getters.

In rare cases, however, these checks turned out to be necessary due
to the problematic design, which needs a long-term solution.

This CL adds those checks back in as a short-term workaround.

Bug: 860626
Change-Id: I78acb597851249a09d00094e6f1984d8c5b76613
Reviewed-on: https://chromium-review.googlesource.com/1138650
Commit-Queue: Hongchan Choi <hongchan@chromium.org>
Reviewed-by: Raymond Toy <rtoy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575449}

third_party/blink/renderer/modules/webaudio/base_audio_context.h

@@ -127,11 +127,19 @@ class MODULES_EXPORT BaseAudioContext
   AudioDestinationNode* destination() const;
 
   size_t CurrentSampleFrame() const {
-    return destination_node_->GetAudioDestinationHandler().CurrentSampleFrame();
+    // TODO(crbug.com/863951): |destination_node| is a GC-mananged object and
+    // should not be touched by the audio rendering thread.
+    return destination_node_ ? destination_node_->GetAudioDestinationHandler()
+                                   .CurrentSampleFrame()
+                             : 0;
   }
 
   double currentTime() const {
-    return destination_node_->GetAudioDestinationHandler().CurrentTime();
+    // TODO(crbug.com/863951): |destination_node| is a GC-mananged object and
+    // should not be touched by the audio rendering thread.
+    return destination_node_
+               ? destination_node_->GetAudioDestinationHandler().CurrentTime()
+               : 0;
   }
 
   float sampleRate() const {