Re-attempt to remove hang watching for deprecated BrowserThreads.

https://chromium-review.googlesource.com/c/chromium/src/+/878825
didn't cause as big of a spike as I expected (~250 instead of ~1000),
will wait for it to settle a bit more before landing this CL
(to confirm whether this one still causes havoc and there's more to
uncover or not)

R=asvitkine@chromium.org

Bug: 804345
Change-Id: Iee2d1f52d55dff2e840420c28e6af7c80d526ca1
Reviewed-on: https://chromium-review.googlesource.com/883122Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532369}

chrome/browser/metrics/thread_watcher.cc

@@ -534,13 +534,6 @@ void ThreadWatcherList::InitializeAndStartWatching(
                 unresponsive_threshold, crash_on_hang_threads);
   StartWatching(BrowserThread::IO, "IO", kSleepTime, kUnresponsiveTime,
                 unresponsive_threshold, crash_on_hang_threads);
-  // TODO(gab): Stop watching deprecated BrowserThreads, crbug.com/768886.
-  StartWatching(BrowserThread::DB, "DB", kSleepTime, kUnresponsiveTime,
-                unresponsive_threshold, crash_on_hang_threads);
-  StartWatching(BrowserThread::FILE, "FILE", kSleepTime, kUnresponsiveTime,
-                unresponsive_threshold, crash_on_hang_threads);
-  StartWatching(BrowserThread::CACHE, "CACHE", kSleepTime, kUnresponsiveTime,
-                unresponsive_threshold, crash_on_hang_threads);
 }
 
 // static