Moving GraphicsContext and dependencies from core to platform.

public/platform/WebThread.h

Index: public/platform/WebThread.h
diff --git a/public/platform/WebThread.h b/public/platform/WebThread.h
index a69d4215ce13087059c076ff3c972c31a1b37742..6bee10fbfebd929305b7c94031e4bf87e545f79b 100644
--- a/public/platform/WebThread.h
+++ b/public/platform/WebThread.h
@@ -33,7 +33,7 @@ namespace blink {
 //
 // Deleting the thread blocks until all pending, non-delayed tasks have been
 // run.
-class WebThread {
+class BLINK_PLATFORM_EXPORT WebThread {
 public:
     class Task {
     public:
@@ -41,7 +41,7 @@ public:
         virtual void run() = 0;
     };
 
-    class TaskObserver {
+    class BLINK_PLATFORM_EXPORT TaskObserver {
     public:
         virtual ~TaskObserver() { }
         virtual void willProcessTask() = 0;