Add unit tests for duration metrics

As a side-effect, move utils::GetMonotonicTime() into the newly added
ClockInterface type.

BUG=None
TEST=Unit tests pass

Change-Id: I972a7e4ba37b63f96348fbeda901697b8ba2fc05
Reviewed-on: https://gerrit.chromium.org/gerrit/48814
Reviewed-by: Chris Sosa <sosa@chromium.org>
Tested-by: David Zeuthen <zeuthen@chromium.org>
Commit-Queue: David Zeuthen <zeuthen@chromium.org>
diff --git a/SConstruct b/SConstruct
index 78e50b6..fe6170d 100644
--- a/SConstruct
+++ b/SConstruct
@@ -247,6 +247,7 @@
                    certificate_checker.cc
                    chrome_browser_proxy_resolver.cc
                    chrome_proxy_resolver.cc
+                   clock.cc
                    connection_manager.cc
                    constants.cc
                    cycle_breaker.cc