Block null Frame object for Worklet instantiation

Worklet requires a valid Frame object, but GetFrame() from the window
can be nullptr. This CL blocks out such case.

Bug: 792108
Change-Id: Ie3fe64c707447956fcfeac687d4df9f5bf7b26d1
Reviewed-on: https://chromium-review.googlesource.com/809299Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Commit-Queue: Hongchan Choi <hongchan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521956}

third_party/WebKit/Source/modules/webaudio/BaseAudioContext.cpp

@@ -151,7 +151,11 @@ void BaseAudioContext::Initialize() {
 
   if (OriginTrials::audioWorkletEnabled(GetExecutionContext()) ||
       RuntimeEnabledFeatures::AudioWorkletEnabled()) {
-    audio_worklet_ = AudioWorklet::Create(this);
+    // Worklet requires a valid Frame object, but GetFrame() from the window
+    // can be nullptr. Block out such case. See: crbug.com/792108
+    if (GetExecutionContext()->ExecutingWindow()->GetFrame()) {
+      audio_worklet_ = AudioWorklet::Create(this);
+    }
   }
 
   if (destination_node_) {