[Android] Add phone progress bar animation behind a flag.

chrome/android/java/src/org/chromium/chrome/browser/widget/ProgressAnimationFastStart.java

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+package org.chromium.chrome.browser.widget;
+
+/**
+ * Progress bar animation logic that starts fast at the beginning and slows down approaching to the
+ * end.
+ */
+class ProgressAnimationFastStart implements ToolbarProgressBar.AnimationLogic {
+    // The speed unit is progress per second where 0 <= progress <= 1.
+    private static final float NORMALIZED_INITIAL_SPEED = 1.5f;
+    private static final float FINISHING_SPEED = 2.0f;
+
+    private float mProgress;
+
+    @Override
+    public void reset() {
+        mProgress = 0.0f;
+    }
+
+    @Override
+    public float updateProgress(float targetProgress, float frameTimeSec, int resolution) {
+        assert mProgress <= targetProgress;
+
+        float progressChange;
+        if (targetProgress == 1.0f) {
+            progressChange = FINISHING_SPEED * frameTimeSec;
+        } else {
+            progressChange = (targetProgress - mProgress)
+                    * (1.0f - (float) Math.exp(-frameTimeSec * NORMALIZED_INITIAL_SPEED));
+        }
+
+        mProgress = Math.min(mProgress + progressChange, targetProgress);
+        if (targetProgress - mProgress < 0.5f / resolution) mProgress = targetProgress;
+
+        return mProgress;
+    }
+}