Add observers for DIP scale change.

ui/android/java/src/org/chromium/ui/base/WindowAndroid.java

Index: ui/android/java/src/org/chromium/ui/base/WindowAndroid.java
diff --git a/ui/android/java/src/org/chromium/ui/base/WindowAndroid.java b/ui/android/java/src/org/chromium/ui/base/WindowAndroid.java
index dabb468983aa8e19a23b3e5ce859786048a0485c..0b2d48488b8d377ab564c5fc5de453675de9c0dc 100644
--- a/ui/android/java/src/org/chromium/ui/base/WindowAndroid.java
+++ b/ui/android/java/src/org/chromium/ui/base/WindowAndroid.java
@@ -71,6 +71,7 @@ public class WindowAndroid {
     // Native pointer to the c++ WindowAndroid object.
     private long mNativeWindowAndroid = 0;
     private final VSyncMonitor mVSyncMonitor;
+    private final DIPScaleMonitor mDIPScaleMonitor;
 
     // A string used as a key to store intent errors in a bundle
     static final String WINDOW_CALLBACK_ERRORS = "window_callback_errors";
@@ -173,6 +174,7 @@ public class WindowAndroid {
         mOutstandingIntents = new SparseArray<IntentCallback>();
         mIntentErrors = new HashMap<Integer, String>();
         mVSyncMonitor = new VSyncMonitor(context, mVSyncListener);
+        mDIPScaleMonitor = new DIPScaleMonitor(this);
         mAccessibilityManager = (AccessibilityManager) mApplicationContext.getSystemService(
                 Context.ACCESSIBILITY_SERVICE);
     }
@@ -514,6 +516,8 @@ public class WindowAndroid {
         if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT) {
             if (mTouchExplorationMonitor != null) mTouchExplorationMonitor.destroy();
         }
+
+        mDIPScaleMonitor.onDestroy();
     }
 
     /**
@@ -642,6 +646,23 @@ public class WindowAndroid {
         });
     }
 
+    // Device scale factor related methods.
+
+    /**
+     * Returns current device scale factor, aka DIP scale or display density.
+     */
+    public float getDIPScale() {
+        return mDIPScaleMonitor.getDIPScale();
+    }
+
+    public void addDIPScaleListener(DIPScaleListener listener) {
+        mDIPScaleMonitor.addObserver(listener);
+    }
+
+    public void removeDIPScaleListener(DIPScaleListener listener) {
+        mDIPScaleMonitor.removeObserver(listener);
+    }
+
     /**
      * Getter for the current context (not necessarily the application context).
      * Make no assumptions regarding what type of Context is returned here, it could be for example