Fix ads not shown if a background tab is playing video

Fixes brave/brave-browser#3779

Author: tmancey

vendor/bat-native-ads/src/bat/ads/internal/ads_impl.cc

@@ -248,7 +248,9 @@ void AdsImpl::OnMediaStopped(const int32_t tab_id) {
 }
 
 bool AdsImpl::IsMediaPlaying() const {
-  if (media_playing_.empty()) {
+  auto tab = media_playing_.find(last_shown_tab_id_);
+  if (tab == media_playing_.end()) {
+    // Media is not playing in the last shown tab
     return false;
   }
 

vendor/bat-native-ads/src/bat/ads/internal/ads_tabs_unittest.cc

@@ -140,6 +140,7 @@ class AdsTabsTest : public ::testing::Test {
 
 TEST_F(AdsTabsTest, Media_IsPlaying) {
   // Arrange
+  ads_->TabUpdated(1, "https://brave.com", true, false);
   ads_->OnMediaPlaying(1);
 
   // Act
@@ -151,6 +152,8 @@ TEST_F(AdsTabsTest, Media_IsPlaying) {
 
 TEST_F(AdsTabsTest, Media_NotPlaying) {
   // Arrange
+  ads_->TabUpdated(1, "https://brave.com", true, false);
+
   ads_->OnMediaPlaying(1);
   ads_->OnMediaPlaying(2);