startup.mobile: stricter battery temperature requirements

SHERIFFS: This could make some benchmarks run faster on continuous bots
          because the devices gets cooled down. The benchmark touched by
          this change is _not_ monitored, but the benchmarks running
          _after_ this one may see a change. This is expected.

Traces from experiment [1] showed that waiting for battery to cool down
to 32C happens every 7 runs adds adds a couple of minutes of waiting and
does not remove the upwards trend on metrics entirely. During those cool
runs the noise was significantly lower than on bots, using the same
hardware (N5X). Trying if this value makes bots bappier.

Cooling down seems unnecessary on N5 and on Go devices, but it would be
good to keep an eye on them anyway and see later how to properly make
this device-dependent in the future, if there is need.

[1] NOT_FOR_COMMIT: Sleep before/after pagecache flush
    https://chromium-review.googlesource.com/c/chromium/src/+/1210643

Bug: 881384
Change-Id: I304437b4231503d7a5d461ee00190bdb8ee5f5cd
Reviewed-on: https://chromium-review.googlesource.com/1217035
Commit-Queue: Ned Nguyen <nednguyen@google.com>
Reviewed-by: Ned Nguyen <nednguyen@google.com>
Cr-Commit-Position: refs/heads/master@{#590039}

tools/perf/benchmarks/startup_mobile.py

@@ -67,7 +67,7 @@ class _MobileStartupSharedState(story_module.SharedState):
       # TODO(crbug.com/811244): Determine whether this ensures the page cache is
       # cleared after |FlushOsPageCaches()| returns.
       self._possible_browser.FlushOsPageCaches()
-    self.platform.WaitForBatteryTemperature(35)
+    self.platform.WaitForBatteryTemperature(32)
     self.platform.StartActivity(
         intent.Intent(package=self._possible_browser.settings.package,
                       activity=self._possible_browser.settings.activity,