[ScopedTaskEnvironment] Add thread annotations to MockTimeDomain::now_ticks_

As a follow-up to
https://chromium-review.googlesource.com/c/chromium/src/+/1686776/14/base/test/scoped_task_environment.cc#366

Doing so I realized that a thread_annotations bug I'd also hit in
TaskTracker is OS_NACL specific. Fixed in
https://chromium-review.googlesource.com/c/chromium/src/+/1695681
and re-enabling proper safety checks in TaskTracker here.

R=alexclarke@chromium.org

Bug: 946657, 982423
Change-Id: Ibbee23cd855f9c124bb41dfd8d2381b03a020132
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1692826
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Alex Clarke <alexclarke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#676063}

base/task/thread_pool/task_tracker.cc

@@ -341,9 +341,7 @@ void TaskTracker::StartShutdown() {
   }
 }
 
-// TODO(gab): Figure out why TS_UNCHECKED_READ is insufficient to make thread
-// analysis of |shutdown_event_| happy on POSIX.
-void TaskTracker::CompleteShutdown() NO_THREAD_SAFETY_ANALYSIS {
+void TaskTracker::CompleteShutdown() {
   // It is safe to access |shutdown_event_| without holding |lock_| because the
   // pointer never changes after being set by StartShutdown(), which must be
   // called before this.

base/test/scoped_task_environment.cc

@@ -217,8 +217,9 @@ class ScopedTaskEnvironment::MockTimeDomain
     if (!run_time)
       return base::nullopt;
 
-    // Check if we have a task that should be running now.
-    if (run_time <= now_ticks_)
+    // Check if we have a task that should be running now. Reading |now_ticks_|
+    // from the main thread doesn't require the lock.
+    if (run_time <= TS_UNCHECKED_READ(now_ticks_))
       return base::TimeDelta();
 
     // The next task is a future delayed task. Since we're using mock time, we
@@ -293,7 +294,8 @@ class ScopedTaskEnvironment::MockTimeDomain
 
   // Only ever written to from the main sequence. Start from real Now() instead
   // of zero to give a more realistic view to tests.
-  TimeTicks now_ticks_{base::subtle::TimeTicksNowIgnoringOverride()};
+  TimeTicks now_ticks_ GUARDED_BY(now_ticks_lock_){
+      base::subtle::TimeTicksNowIgnoringOverride()};
 };
 
 ScopedTaskEnvironment::MockTimeDomain*