Add observers for DIP scale change.

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

Index: ui/android/java/src/org/chromium/ui/display/DisplayAndroid.java
diff --git a/ui/android/java/src/org/chromium/ui/display/DisplayAndroid.java b/ui/android/java/src/org/chromium/ui/display/DisplayAndroid.java
index 0702a70def1c1fdbe7bc2afb60031bec76c506cf..fe156a6540ebfafd4a7e31559cd55b2ab1c7dca6 100644
--- a/ui/android/java/src/org/chromium/ui/display/DisplayAndroid.java
+++ b/ui/android/java/src/org/chromium/ui/display/DisplayAndroid.java
@@ -36,6 +36,13 @@ public class DisplayAndroid {
          * @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";
@@ -249,6 +256,8 @@ public class DisplayAndroid {
     @SuppressWarnings("deprecation")
     @TargetApi(Build.VERSION_CODES.JELLY_BEAN_MR1)
     /* package */ void updateFromDisplay(Display display) {
+        float oldDensity = mDisplayMetrics.density;
+
         display.getSize(mSize);
         display.getMetrics(mDisplayMetrics);
 
@@ -273,6 +282,16 @@ public class DisplayAndroid {
                 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() {