Refactor TimeBase to post tasks. Workers to use real Idle tasks.

Source/core/fetch/ResourceTest.cpp

Index: Source/core/fetch/ResourceTest.cpp
diff --git a/Source/core/fetch/ResourceTest.cpp b/Source/core/fetch/ResourceTest.cpp
index 9cd3cf1fa408f0209b66bc06c8075b48fdc32d28..8dcc647956a6cdb2087870a744b09523e163f724 100644
--- a/Source/core/fetch/ResourceTest.cpp
+++ b/Source/core/fetch/ResourceTest.cpp
@@ -22,7 +22,7 @@ namespace {
 
 class MockPlatform final : public Platform {
 public:
-    MockPlatform() { }
+    MockPlatform() : m_oldPlatform(Platform::current()) { }
     ~MockPlatform() override { }
 
     // From blink::Platform:
@@ -40,7 +40,13 @@ public:
         return m_cachedURLs;
     }
 
+    WebThread* currentThread() override
+    {
+        return m_oldPlatform->currentThread();
+    }
+
 private:
+    Platform* m_oldPlatform; // NOT OWNED.
     Vector<WebURL> m_cachedURLs;
 };