Disable ThreadWatcher in STABLE channel temporarily to measure its

impact on battery.

Will be measuring its impact in the following dashboard.

https://chromeperf.appspot.com/report?masters=ChromiumPerf&bots=chromium-rel-mac9&tests=power.top_25%2Fidle_wakeups_total&checked=idle_wakeups_total%2Cref

R=asvitkine@chromium.org
BUG=426203

Review URL: https://codereview.chromium.org/1107283005

Cr-Commit-Position: refs/heads/master@{#327555}

chrome/browser/metrics/thread_watcher.cc

@@ -542,6 +542,7 @@ void ThreadWatcherList::InitializeAndStartWatching(
   // Disable ThreadWatcher in Canary channel.
   chrome::VersionInfo::Channel channel = chrome::VersionInfo::GetChannel();
   if (channel == chrome::VersionInfo::CHANNEL_CANARY ||
+      channel == chrome::VersionInfo::CHANNEL_STABLE ||
       channel == chrome::VersionInfo::CHANNEL_UNKNOWN) {
     return;
   }