Use SimpleTestClock::Advance in WallClockTimerTest

This is minor simplification in WallClockTimer: Instead of SetNow(Now()
+ delay) idiom use special SimpleTestClock method.

Bug: None
Change-Id: I035bfe44792c0e12c23373ded65ad520792d1824
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2418317
Commit-Queue: Oleg Davydov <burunduk@chromium.org>
Commit-Queue: Owen Min <zmin@chromium.org>
Auto-Submit: Oleg Davydov <burunduk@chromium.org>
Reviewed-by: Owen Min <zmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808381}

base/util/timer/wall_clock_timer_unittest.cc

@@ -27,7 +27,7 @@ class WallClockTimerTest : public ::testing::Test {
     if (!with_power)
       fake_power_monitor_source_.Suspend();
 
-    clock_.SetNow(clock_.Now() + delay);
+    clock_.Advance(delay);
 
     if (with_power) {
       task_environment_.FastForwardBy(delay);