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

Source/core/fetch/CachingCorrectnessTest.cpp

Index: Source/core/fetch/CachingCorrectnessTest.cpp
diff --git a/Source/core/fetch/CachingCorrectnessTest.cpp b/Source/core/fetch/CachingCorrectnessTest.cpp
index b729004a7782da1ba3c50864f2a29f05119eb7d0..474f94f7d0947fe6d371664331a92e336d7ea618 100644
--- a/Source/core/fetch/CachingCorrectnessTest.cpp
+++ b/Source/core/fetch/CachingCorrectnessTest.cpp
@@ -124,7 +124,7 @@ private:
     // A simple platform that mocks out the clock, for cache freshness testing.
     class ProxyPlatform : public blink::Platform {
     public:
-        ProxyPlatform() : m_elapsedSeconds(0.) { }
+        explicit ProxyPlatform() : m_oldPlatform(blink::Platform::current()), m_elapsedSeconds(0.) { }
 
         void advanceClock(double seconds)
         {
@@ -145,6 +145,12 @@ private:
             return &kAConstUnsignedCharZero;
         }
 
+        WebThread* currentThread() override
+        {
+            return m_oldPlatform->currentThread();
+        }
+
+        blink::Platform* m_oldPlatform; // NOT OWNED
         double m_elapsedSeconds;
     };