Use std::deque to store the stack of currently executing tasks

The stack of currently executing stacks includes a PendingTask field. A
pointer to this field is stored in TLS. However, std::vector does not
guarantee pointer stability on resize.

Bug: 1064891
Change-Id: I04eb06c9521722f08fd72826f552cedaffe61b53
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2146349
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759017}

base/task/sequence_manager/sequence_manager_impl.cc

@@ -44,13 +44,6 @@ GetTLSSequenceManagerImpl() {
 
 }  // namespace
 
-// This controls how big the the initial for
-// |MainThreadOnly::task_execution_stack| should be. We don't expect to see
-// depths of more than 2 unless cooperative scheduling is used on Blink, where
-// we might get up to 6. Anyway 10 was chosen because it's a round number
-// greater than current anticipated usage.
-static constexpr const size_t kInitialTaskExecutionStackReserveCount = 10;
-
 std::unique_ptr<SequenceManager> CreateSequenceManagerOnCurrentThread(
     SequenceManager::Settings settings) {
   return internal::SequenceManagerImpl::CreateOnCurrentThread(
@@ -252,7 +245,6 @@ SequenceManagerImpl::MainThreadOnly::MainThreadOnly(
     random_generator = std::mt19937_64(RandUint64());
     uniform_distribution = std::uniform_real_distribution<double>(0.0, 1.0);
   }
-  task_execution_stack.reserve(kInitialTaskExecutionStackReserveCount);
 }
 
 SequenceManagerImpl::MainThreadOnly::~MainThreadOnly() = default;

base/task/sequence_manager/sequence_manager_impl.h

@@ -5,6 +5,7 @@
 #ifndef BASE_TASK_SEQUENCE_MANAGER_SEQUENCE_MANAGER_IMPL_H_
 #define BASE_TASK_SEQUENCE_MANAGER_SEQUENCE_MANAGER_IMPL_H_
 
+#include <deque>
 #include <list>
 #include <map>
 #include <memory>
@@ -12,7 +13,6 @@
 #include <set>
 #include <unordered_map>
 #include <utility>
-#include <vector>
 
 #include "base/atomic_sequence_num.h"
 #include "base/cancelable_callback.h"
@@ -302,7 +302,9 @@ class BASE_EXPORT SequenceManagerImpl
     bool nesting_observer_registered_ = false;
 
     // Due to nested runloops more than one task can be executing concurrently.
-    std::vector<ExecutingTask> task_execution_stack;
+    // Note that this uses std::deque for pointer stability, since pointers to
+    // objects in this container are stored in TLS.
+    std::deque<ExecutingTask> task_execution_stack;
 
     Observer* observer = nullptr;  // NOT OWNED