Implement async AbortAll for the compiler dispatcher

src/cancelable-task.h

Index: src/cancelable-task.h
diff --git a/src/cancelable-task.h b/src/cancelable-task.h
index 578db6f6b997c624a0c3a7af901978f6fd534de7..7636bf6f13c7dd3ff6c9c0aea2070612b989422f 100644
--- a/src/cancelable-task.h
+++ b/src/cancelable-task.h
@@ -45,6 +45,17 @@ class V8_EXPORT_PRIVATE CancelableTaskManager {
   // already running. This disallows subsequent Register calls.
   void CancelAndWait();
 
+  // Tries to cancel all remaining registered tasks. The return value indicates
+  // whether
+  //
+  // 1) No tasks were registered (kTaskRemoved), or
+  //
+  // 2) There is at least one remaining task that couldn't be cancled
+  // (kTaskRunning), or
+  //
+  // 3) All registered tasks were cancelled (kTaskAborted).
+  TryAbortResult Cancel();

[marja, 2017/01/04 12:54:43]

The naming is confusing but it's there because of this CL...

We should either have:
TryAbort
TryAbortAllAndWait (now CancelAndWait)
TryAbortAll (now Cancel)

or:

Cancel (now TryAbort)
CancelAllAndWait (now CancelAndWait)
CancelAll (now Cancel)

wdyt? (But this probably shouldn't be fixed by this CL?)

[jochen (gone - plz use gerrit), 2017/01/04 13:18:31]

what about just renaming Cancel to TryAbortAll? CancelAll() doesn't try, it just
does - but the other two just try their best

+
  private:
   // Only called by {Cancelable} destructor. The task is done with executing,
   // but needs to be removed.