Revert "Fix thread checks in HangWatcher"

This reverts commit 07c000ce.

Reason for revert: use-of-uninitialized-value is detected in Linux MSan Tests.
https://ci.chromium.org/p/chromium/builders/ci/Linux%20MSan%20Tests/25981

==29008==WARNING: MemorySanitizer: use-of-uninitialized-value
#0 0x5580a100b292 in base::HangWatcher::GetTimeSinceLastCriticalMemoryPressureCrashKey() ./../../base/threading/hang_watcher.cc:268:7


Original change's description:
> Fix thread checks in HangWatcher
>
> Remove the thread check that would never pass and make the variable
> that is used by multiple threads thread safe.
>
> Bug: 1142155
> Change-Id: Iae33e15733eb78a47a664515e1abec671edbb64d
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2495579
> Reviewed-by: François Doray <fdoray@chromium.org>
> Commit-Queue: Oliver Li <olivierli@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#820817}

TBR=fdoray@chromium.org,olivierli@chromium.org

Change-Id: I463c2928c91878d396a0dd7ebb93b3c625e26a56
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1142155
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2500385Reviewed-by: Kunihiko Sakamoto <ksakamoto@chromium.org>
Commit-Queue: Kunihiko Sakamoto <ksakamoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821004}

base/threading/hang_watcher.cc

@@ -264,9 +264,7 @@ HangWatcher::GetTimeSinceLastCriticalMemoryPressureCrashKey() {
   static debug::CrashKeyString* crash_key = AllocateCrashKeyString(
       "seconds-since-last-memory-pressure", kCrashKeyContentSize);
 
-  const base::TimeTicks last_critical_memory_pressure_time =
-      last_critical_memory_pressure_.load(std::memory_order_relaxed);
-  if (last_critical_memory_pressure_time.is_null()) {
+  if (last_critical_memory_pressure_.is_null()) {
     constexpr char kNoMemoryPressureMsg[] = "No critical memory pressure";
     static_assert(
         base::size(kNoMemoryPressureMsg) <=
@@ -275,7 +273,7 @@ HangWatcher::GetTimeSinceLastCriticalMemoryPressureCrashKey() {
     return debug::ScopedCrashKeyString(crash_key, kNoMemoryPressureMsg);
   } else {
     base::TimeDelta time_since_last_critical_memory_pressure =
-        base::TimeTicks::Now() - last_critical_memory_pressure_time;
+        base::TimeTicks::Now() - last_critical_memory_pressure_;
     return debug::ScopedCrashKeyString(
         crash_key, base::NumberToString(
                        time_since_last_critical_memory_pressure.InSeconds()));
@@ -285,10 +283,11 @@ HangWatcher::GetTimeSinceLastCriticalMemoryPressureCrashKey() {
 
 void HangWatcher::OnMemoryPressure(
     base::MemoryPressureListener::MemoryPressureLevel memory_pressure_level) {
+  DCHECK_CALLED_ON_VALID_THREAD(hang_watcher_thread_checker_);
+
   if (memory_pressure_level ==
       base::MemoryPressureListener::MEMORY_PRESSURE_LEVEL_CRITICAL) {
-    last_critical_memory_pressure_.store(base::TimeTicks::Now(),
-                                         std::memory_order_relaxed);
+    last_critical_memory_pressure_ = base::TimeTicks::Now();
   }
 }
 

base/threading/hang_watcher.h

@@ -334,9 +334,8 @@ class BASE_EXPORT HangWatcher : public DelegateSimpleThread::Delegate {
   base::MemoryPressureListener memory_pressure_listener_;
 
   // The last time at which a critical memory pressure signal was received, or
-  // null if no signal was ever received. Atomic because it's set and read from
-  // different threads.
-  std::atomic<base::TimeTicks> last_critical_memory_pressure_;
+  // null if no signal was ever received.
+  base::TimeTicks last_critical_memory_pressure_;
 
   // The time after which all deadlines in |watch_states_| need to be for a hang
   // to be reported.