Remove non-monotonic clock support

Real time clock may cause problems as they can move (even backwards) if
the clock is changed, eg updated by NTP.

Non-monotonic clocks still in use on some platform (I'm looking at you,
Apple) for timed waits, but that should be less of an issue than actual
timestamps.

BUG=webrtc:5452

Review URL: https://codereview.webrtc.org/1613013002

Cr-Commit-Position: refs/heads/master@{#11375}
diff --git a/webrtc/base/platform_thread.cc b/webrtc/base/platform_thread.cc
index 05b7a25..b6fd873 100644
--- a/webrtc/base/platform_thread.cc
+++ b/webrtc/base/platform_thread.cc
@@ -179,7 +179,7 @@
   if (!name_.empty())
     rtc::SetCurrentThreadName(name_.c_str());
   do {
-    // The interface contract of Start/Stop is that for a successfull call to
+    // The interface contract of Start/Stop is that for a successful call to
     // Start, there should be at least one call to the run function.  So we
     // call the function before checking |stop_|.
     if (!run_function_(obj_))