base/task: Add debug annotation to cancelled task sweeping

We're seeing some crashes during removing cancelled tasks from work
queues. Add some debug annotations to help understand where these
invalid tasks are coming from.

Bug: 1071475
Change-Id: Ic4ba31b8a9585e44e70c7151f461615253291ea1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2153117
Commit-Queue: Sami Kyöstilä <skyostil@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Auto-Submit: Sami Kyöstilä <skyostil@chromium.org>
Cr-Commit-Position: refs/heads/master@{#760240}

base/task/sequence_manager/work_queue.cc

@@ -4,8 +4,10 @@
 
 #include "base/task/sequence_manager/work_queue.h"
 
+#include "base/debug/alias.h"
 #include "base/task/sequence_manager/sequence_manager_impl.h"
 #include "base/task/sequence_manager/work_queue_sets.h"
+#include "build/build_config.h"
 
 namespace base {
 namespace sequence_manager {
@@ -200,8 +202,31 @@ bool WorkQueue::RemoveAllCanceledTasksFromFront() {
   if (!work_queue_sets_)
     return false;
   bool task_removed = false;
-  while (!tasks_.empty() &&
-         (!tasks_.front().task || tasks_.front().task.IsCancelled())) {
+  while (!tasks_.empty()) {
+    const auto& pending_task = tasks_.front();
+#if !defined(OS_NACL)
+    // Record some debugging information about the task.
+    // TODO(skyostil): Remove once crbug.com/1071475 is resolved.
+    DEBUG_ALIAS_FOR_CSTR(debug_file_name,
+                         pending_task.posted_from.file_name()
+                             ? pending_task.posted_from.file_name()
+                             : "",
+                         16);
+    DEBUG_ALIAS_FOR_CSTR(debug_function_name,
+                         pending_task.posted_from.function_name()
+                             ? pending_task.posted_from.function_name()
+                             : "",
+                         16);
+    int debug_line_number = pending_task.posted_from.line_number();
+    const void* debug_pc = pending_task.posted_from.program_counter();
+    const void* debug_bind_state =
+        reinterpret_cast<const void*>(&pending_task.task);
+    base::debug::Alias(&debug_line_number);
+    base::debug::Alias(&debug_pc);
+    base::debug::Alias(&debug_bind_state);
+#endif  // !defined(OS_NACL)
+    if (pending_task.task && !pending_task.task.IsCancelled())
+      break;
     tasks_.pop_front();
     task_removed = true;
   }