browser(firefox): do not complain about SnapshotListener being cleared on Destroy (microsoft#3298)

Author: yury-s

browser_patches/firefox/BUILD_NUMBER

@@ -1,2 +1,2 @@
-1150
-Changed: [email protected] Mon Aug  3 19:37:50 PDT 2020
+1151
+Changed: [email protected] Tue Aug  4 16:50:29 PDT 2020

browser_patches/firefox/patches/bootstrap.diff

@@ -2088,7 +2088,7 @@ index 7f91de9e67d7ffa02de3eef1d760e5cfd05e7ad6..37b0320f3bde99ef7635c71452a3a4b7
 
  }  // namespace widget
 diff --git a/widget/headless/HeadlessWidget.cpp b/widget/headless/HeadlessWidget.cpp
-index 7589d8a1a886dab5431e423d20f7d0aa19c2af75..b14925a7b6972c120a59f70fbbe70a1145a5cb4e 100644
+index 7589d8a1a886dab5431e423d20f7d0aa19c2af75..05c79e0d5ee58c45a75b55a65423466be11596fa 100644
 --- a/widget/headless/HeadlessWidget.cpp
 +++ b/widget/headless/HeadlessWidget.cpp
 @@ -104,6 +104,8 @@ void HeadlessWidget::Destroy() {
@@ -2100,13 +2100,14 @@ index 7589d8a1a886dab5431e423d20f7d0aa19c2af75..b14925a7b6972c120a59f70fbbe70a11
    nsBaseWidget::OnDestroy();
 
    nsBaseWidget::Destroy();
-@@ -499,5 +501,13 @@ nsresult HeadlessWidget::SynthesizeNativeTouchPoint(
+@@ -499,5 +501,14 @@ nsresult HeadlessWidget::SynthesizeNativeTouchPoint(
    return NS_OK;
  }
 
 +void HeadlessWidget::SetSnapshotListener(SnapshotListener&& listener) {
 +  if (!mCompositorWidget) {
-+    fprintf(stderr, "Trying to set SnapshotListener without compositor widget\n");
++    if (listener)
++      fprintf(stderr, "Trying to set SnapshotListener without compositor widget\n");
 +    return;
 +  }
 +  mCompositorWidget->SetSnapshotListener(std::move(listener));