Add observers for DIP scale change.

ui/android/java/src/org/chromium/ui/display/DisplayAndroid.java

 // Copyright 2016 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.ui.display;
 
 import android.annotation.TargetApi;
 import android.content.Context;
 import android.graphics.PixelFormat;
 import android.graphics.Point;
@@ skipping 18 matching lines @@
     /**
      * DisplayAndroidObserver interface for changes to this Display.
      */
     public interface DisplayAndroidObserver {
         /**
          * Called whenever the screen orientation changes.
          *
          * @param orientation One of Surface.ROTATION_* values.
          */
         void onRotationChanged(int rotation);
+
+        /**
+         * Called whenever the screen density changes.
+         *
+         * @param screen density, aka Density Independent Pixel scale.
+         */
+        void onDIPScaleChanged(float dipScale);
     }
 
     private static final String TAG = "DisplayAndroid";
 
     private static final DisplayAndroidObserver[] EMPTY_OBSERVER_ARRAY =
             new DisplayAndroidObserver[0];
 
     private final int mSdkDisplayId;
     private final WeakHashMap<DisplayAndroidObserver, Object /* null */> mObservers;
     // Do NOT add strong references to objects with potentially complex lifetime, like Context.
@@ skipping 193 matching lines @@
         mSize = new Point();
         mPhysicalSize = new Point();
         mDisplayMetrics = new DisplayMetrics();
         mPixelFormatId = PixelFormat.RGBA_8888;
         updateFromDisplay(display);
     }
 
     @SuppressWarnings("deprecation")
     @TargetApi(Build.VERSION_CODES.JELLY_BEAN_MR1)
     /* package */ void updateFromDisplay(Display display) {
+        float oldDensity = mDisplayMetrics.density;
+
         display.getSize(mSize);
         display.getMetrics(mDisplayMetrics);
 
         if (hasForcedDIPScale()) mDisplayMetrics.density = sForcedDIPScale.floatValue();
 
         if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.JELLY_BEAN_MR1) {
             display.getRealSize(mPhysicalSize);
         }
 
         // JellyBean MR1 and later always uses RGBA_8888.
         if (Build.VERSION.SDK_INT < Build.VERSION_CODES.JELLY_BEAN_MR1) {
             mPixelFormatId = display.getPixelFormat();
         }
 
         int newRotation = display.getRotation();
         boolean rotationChanged = newRotation != mRotation;
         mRotation = newRotation;
 
         if (rotationChanged) {
             DisplayAndroidObserver[] observers = getObservers();
             for (DisplayAndroidObserver o : observers) {
                 o.onRotationChanged(mRotation);
             }
         }
+
+        // Intentional comparison of floats: we assume that if scales differ,
+        // they differ significantly.
+        boolean dipScaleChanged = oldDensity != mDisplayMetrics.density;
+        if (dipScaleChanged) {
+            DisplayAndroidObserver[] observers = getObservers();
+            for (DisplayAndroidObserver o : observers) {
+                o.onDIPScaleChanged(mDisplayMetrics.density);
+            }
+        }
     }
 
     private DisplayAndroidObserver[] getObservers() {
         // Makes a copy to allow concurrent edit.
         return mObservers.keySet().toArray(EMPTY_OBSERVER_ARRAY);
     }
 }