Add observers for DIP scale change.

ui/android/java/src/org/chromium/ui/gfx/DeviceDisplayInfo.java

Index: ui/android/java/src/org/chromium/ui/gfx/DeviceDisplayInfo.java
diff --git a/ui/android/java/src/org/chromium/ui/gfx/DeviceDisplayInfo.java b/ui/android/java/src/org/chromium/ui/gfx/DeviceDisplayInfo.java
index 44c599042ef3956501203b767b69873a743b6b4d..5ca6ca037e47b45e9f9ae5dbd0e6bd64bbfd109b 100644
--- a/ui/android/java/src/org/chromium/ui/gfx/DeviceDisplayInfo.java
+++ b/ui/android/java/src/org/chromium/ui/gfx/DeviceDisplayInfo.java
@@ -14,6 +14,7 @@ import android.view.Display;
 import android.view.Surface;
 import android.view.WindowManager;
 
+import org.chromium.base.CommandLine;
 import org.chromium.base.annotations.CalledByNative;
 import org.chromium.base.annotations.JNINamespace;
 
@@ -32,6 +33,28 @@ public class DeviceDisplayInfo {
     private Point mTempPoint = new Point();
     private DisplayMetrics mTempMetrics = new DisplayMetrics();
 
+    // The value -1.0 signals that it is unknown whether the forced value is set,
+    // 0.0 means that it is not set and positive that it's set.
+    private static float sForcedDIPScale = -1.0f;
+
+    private static boolean hasForcedDIPScale() {
+        if (sForcedDIPScale < 0) {
+            String forcedScaleAsString = CommandLine.getInstance().getSwitchValue(
+                    DisplaySwitches.FORCE_DEVICE_SCALE_FACTOR);
+            if (forcedScaleAsString == null) {
+                sForcedDIPScale = 0.0f;
+            } else {
+                try {
+                    sForcedDIPScale = Math.abs(Float.valueOf(forcedScaleAsString));
+                } catch (NumberFormatException e) {
+                    // Invalid numerical value is discarded.
+                    sForcedDIPScale = 0.0f;
+                }
+            }
+        }
+        return sForcedDIPScale > 0;
+    }
+
     private DeviceDisplayInfo(Context context) {
         mAppContext = context.getApplicationContext();
         mWinManager = (WindowManager) mAppContext.getSystemService(Context.WINDOW_SERVICE);
@@ -144,6 +167,8 @@ public class DeviceDisplayInfo {
      */
     @CalledByNative
     public double getDIPScale() {
+        if (hasForcedDIPScale()) return sForcedDIPScale;
+
         getDisplay().getMetrics(mTempMetrics);
         return mTempMetrics.density;
     }