Worker: Remove unnecessary null check for Agent

After https://crrev.com/c/2164926, ExecutionContext::GetAgent() does not
return a null Agent instance anymore. This CL removes an unnecessary
null check in WorkerThread class.

Bug: 1074766
Change-Id: I08020af4507dc78d4cfe9236f62549c359146e5e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2263433Reviewed-by: Keishi Hattori <keishi@chromium.org>
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#781862}

third_party/blink/renderer/core/execution_context/execution_context.cc

@@ -64,7 +64,9 @@ ExecutionContext::ExecutionContext(v8::Isolate* isolate, Agent* agent)
       is_context_destroyed_(false),
       csp_delegate_(MakeGarbageCollected<ExecutionContextCSPDelegate>(*this)),
       window_interaction_tokens_(0),
-      referrer_policy_(network::mojom::ReferrerPolicy::kDefault) {}
+      referrer_policy_(network::mojom::ReferrerPolicy::kDefault) {
+  DCHECK(agent_);
+}
 
 ExecutionContext::~ExecutionContext() = default;
 

third_party/blink/renderer/core/execution_context/execution_context.h

@@ -362,7 +362,7 @@ class CORE_EXPORT ExecutionContext : public Supplementable<ExecutionContext>,
 
   v8::Isolate* const isolate_;
 
-  Member<Agent> agent_;
+  const Member<Agent> agent_;
 
   bool DispatchErrorEventInternal(ErrorEvent*, SanitizeScriptErrors);
 

third_party/blink/renderer/core/workers/worker_thread.cc

@@ -321,11 +321,7 @@ void WorkerThread::DidProcessTask(const base::PendingTask& pending_task) {
 
   // TODO(tzik): Move this to WorkerThreadScheduler::OnTaskCompleted(), so that
   // metrics for microtasks are counted as a part of the preceding task.
-  // TODO(nhiroki): Replace this null check with DCHECK(agent) after making
-  // WorkletGlobalScope take a proper Agent.
-  if (Agent* agent = GlobalScope()->GetAgent()) {
-    agent->event_loop()->PerformMicrotaskCheckpoint();
-  }
+  GlobalScope()->GetAgent()->event_loop()->PerformMicrotaskCheckpoint();
 
   // Microtask::PerformCheckpoint() runs microtasks and its completion hooks for
   // the default microtask queue. The default queue may contain the microtasks