Add a BrowserThread::DCheckCurrentlyOn function.

content/public/browser/browser_thread.h

Index: content/public/browser/browser_thread.h
diff --git a/content/public/browser/browser_thread.h b/content/public/browser/browser_thread.h
index 55cb62b22355d339822414f06874d2bd2695590f..4054fe079744120ef874b6d012893b85c470488b 100644
--- a/content/public/browser/browser_thread.h
+++ b/content/public/browser/browser_thread.h
@@ -10,15 +10,12 @@
 #include "base/basictypes.h"
 #include "base/callback.h"
 #include "base/location.h"
+#include "base/logging.h"
 #include "base/message_loop/message_loop_proxy.h"
 #include "base/task_runner_util.h"
 #include "base/time/time.h"
 #include "content/common/content_export.h"
 
-#if defined(UNIT_TEST)
-#include "base/logging.h"
-#endif  // UNIT_TEST
-
 namespace base {
 class MessageLoop;
 class SequencedWorkerPool;
@@ -30,6 +27,13 @@ namespace content {
 class BrowserThreadDelegate;
 class BrowserThreadImpl;
 
+// Use DCHECK_CURRENTLY_ON(BrowserThread::ID) to assert that a function can only
+// be called on the named BrowserThread.
+#define DCHECK_CURRENTLY_ON(thread_identifier)                      \
+  (DCHECK(::content::BrowserThread::CurrentlyOn(thread_identifier)) \
+   << ::content::BrowserThread::GetDCheckCurrentlyOnErrorMessage(   \
+          thread_identifier))
+
 ///////////////////////////////////////////////////////////////////////////////
 // BrowserThread
 //
@@ -188,7 +192,7 @@ class CONTENT_EXPORT BrowserThread {
   static bool IsThreadInitialized(ID identifier);
 
   // Callable on any thread.  Returns whether you're currently on a particular
-  // thread.
+  // thread.  To DCHECK this, use the DCHECK_CURRENTLY_ON() macro above.
   static bool CurrentlyOn(ID identifier);
 
   // Callable on any thread.  Returns whether the threads message loop is valid.
@@ -266,6 +270,9 @@ class CONTENT_EXPORT BrowserThread {
   struct DeleteOnFileThread : public DeleteOnThread<FILE> { };
   struct DeleteOnDBThread : public DeleteOnThread<DB> { };
 
+  // Returns an appropriate error message for when DCHECK_CURRENTLY_ON() fails.
+  static std::string GetDCheckCurrentlyOnErrorMessage(ID expected);
+
  private:
   friend class BrowserThreadImpl;