Move timer functionality from util/ to common/

The main timer functionality (get absolute time) is moved to common/ for
use in ANGLE itself (in upcoming overlay change).  util/Timer.h is no
longer an abstract class and uses this functionality to implement the
timer.

Bug: angleproject:3757
Change-Id: I3fe418778d80d1089c9bfe43a9e8098e43236f18
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1769061
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>

util/Timer.cpp

@@ -0,0 +1,40 @@
+//
+// Copyright 2019 The ANGLE Project Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+//
+// Timer.cpp: Implementation of a high precision timer class.
+//
+
+#include "util/Timer.h"
+
+#include "common/system_utils.h"
+
+Timer::Timer() : mRunning(false), mStartTime(0), mStopTime(0) {}
+
+void Timer::start()
+{
+    mStartTime = angle::GetCurrentTime();
+    mRunning   = true;
+}
+
+void Timer::stop()
+{
+    mStopTime = angle::GetCurrentTime();
+    mRunning  = false;
+}
+
+double Timer::getElapsedTime() const
+{
+    double endTime;
+    if (mRunning)
+    {
+        endTime = angle::GetCurrentTime();
+    }
+    else
+    {
+        endTime = mStopTime;
+    }
+
+    return endTime - mStartTime;
+}