Reland "Fix thread checks in HangWatcher"

This is a reland of 07c000ce

This CL fixes use of initialized memory which was detected by msan.

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}

Bug: 1142155
Change-Id: I9b642bacbf8b801cd3a549c642c1687449edab86
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2502748Reviewed-by: François Doray <fdoray@chromium.org>
Commit-Queue: Oliver Li <olivierli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821385}

base/threading/hang_watcher.cc

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

base/threading/hang_watcher.h

@@ -334,8 +334,10 @@ 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.
-  base::TimeTicks last_critical_memory_pressure_;
+  // 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_{
+      base::TimeTicks()};
 
   // The time after which all deadlines in |watch_states_| need to be for a hang
   // to be reported.