[base] Don't sample Now() in POSIX's WaitableEvent::Wait()

Fixes a race between sampling Now() on an idle content_unittests helper
thread and ~ScopedTimeClockOverrides. TSAN log:
https://paste.googleplex.com/6136256530481152

Attempt to avoid requiring TSAN suppression @
https://chromium-review.googlesource.com/c/chromium/src/+/1488877

R=kylechar@chromium.org

Bug: 910524
Change-Id: Ida3a3d8fe689a3d51a6f6f6d6870e2dca443f5d1
Reviewed-on: https://chromium-review.googlesource.com/c/1489297Reviewed-by: kylechar <kylechar@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635615}

base/synchronization/waitable_event_posix.cc

@@ -200,9 +200,12 @@ bool WaitableEvent::TimedWaitUntil(const TimeTicks& end_time) {
   // again before unlocking it.
 
   for (;;) {
-    const TimeTicks current_time(TimeTicks::Now());
+    // Only sample Now() if waiting for a |finite_time|.
+    Optional<TimeTicks> current_time;
+    if (finite_time)
+      current_time = TimeTicks::Now();
 
-    if (sw.fired() || (finite_time && current_time >= end_time)) {
+    if (sw.fired() || (finite_time && *current_time >= end_time)) {
       const bool return_value = sw.fired();
 
       // We can't acquire @lock_ before releasing the SyncWaiter lock (because
@@ -226,7 +229,7 @@ bool WaitableEvent::TimedWaitUntil(const TimeTicks& end_time) {
     }
 
     if (finite_time) {
-      const TimeDelta max_wait(end_time - current_time);
+      const TimeDelta max_wait(end_time - *current_time);
       sw.cv()->TimedWait(max_wait);
     } else {
       sw.cv()->Wait();