Bug 1587622 - Use MOZ_ASSERT_UNREACHABLE() instead of double-conversion's UNREACHABLE() in DOM media code. r=karlt

Differential Revision: https://phabricator.services.mozilla.com/D48880

--HG--
extra : moz-landing-system : lando

dom/html/HTMLMediaElement.cpp

@@ -1715,7 +1715,9 @@ already_AddRefed<Promise> HTMLMediaElement::MozRequestDebugInfo(
             [promise, ptr = std::move(result)]() {
               promise->MaybeResolve(ptr.get());
             },
-            []() { UNREACHABLE(); });
+            []() {
+              MOZ_ASSERT_UNREACHABLE("Unexpected RequestDebugInfo() rejection");
+            });
   } else {
     promise->MaybeResolve(result.get());
   }

dom/media/MediaDecoder.cpp

@@ -1356,7 +1356,7 @@ RefPtr<GenericPromise> MediaDecoder::RequestDebugInfo(
           SystemGroup::AbstractMainThreadFor(TaskCategory::Other), __func__,
           []() { return GenericPromise::CreateAndResolve(true, __func__); },
           []() {
-            UNREACHABLE();
+            MOZ_ASSERT_UNREACHABLE("Unexpected RequestDebugInfo() rejection");
             return GenericPromise::CreateAndResolve(false, __func__);
           });
 }