BatteryManager.getBattery(): gracefully bail on context-detached use.

Source/modules/battery/BatteryManager.cpp

Index: Source/modules/battery/BatteryManager.cpp
diff --git a/Source/modules/battery/BatteryManager.cpp b/Source/modules/battery/BatteryManager.cpp
index e2a9c77466eca0d935e91a049377cb9c1ec89f16..79d8e262aa58b9069bc3427b93a94a5305a103cb 100644
--- a/Source/modules/battery/BatteryManager.cpp
+++ b/Source/modules/battery/BatteryManager.cpp
@@ -43,9 +43,8 @@ ScriptPromise BatteryManager::startRequest(ScriptState* scriptState)
     m_resolver = ScriptPromiseResolver::create(scriptState);
     ScriptPromise promise = m_resolver->promise();
 
-    ASSERT(executionContext());
     // If the context is in a stopped state already, do not start updating.
-    if (m_state == Resolved || executionContext()->activeDOMObjectsAreStopped()) {
+    if (m_state == Resolved || !executionContext() || executionContext()->activeDOMObjectsAreStopped()) {
         // FIXME: Consider returning the same promise in this case. See crbug.com/385025.
         m_state = Resolved;
         m_resolver->resolve(this);