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..30b64790d761e9e6945cd1ed93a3c195fb5d8360 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);
     }
@@ -642,6 +644,23 @@ 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 addDisplayObserver(DisplayObserver observer) {

[boliu, 2016/09/15 05:27:41]

probably should add a note that this observer is on Window rather than "Display"
because we don't have an abstraction for display in java yet

[Tima Vaisburd, 2016/09/16 23:59:26]

Said something in the // comment above..

+        mDIPScaleMonitor.addObserver(observer);
+    }
+
+    public void removeDisplayObserver(DisplayObserver observer) {
+        mDIPScaleMonitor.removeObserver(observer);
+    }
+
     /**
      * 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