[webnfc] Implement watch method for Android nfc mojo service.

device/nfc/android/java/src/org/chromium/device/nfc/NfcTagHandler.java

Index: device/nfc/android/java/src/org/chromium/device/nfc/NfcTagHandler.java
diff --git a/device/nfc/android/java/src/org/chromium/device/nfc/NfcTagHandler.java b/device/nfc/android/java/src/org/chromium/device/nfc/NfcTagHandler.java
index a7adab7047e2b3b879f0a905dbd9050c965f2433..96508aa25dbae7ab9d0c9ef1311bd8275f6e28d2 100644
--- a/device/nfc/android/java/src/org/chromium/device/nfc/NfcTagHandler.java
+++ b/device/nfc/android/java/src/org/chromium/device/nfc/NfcTagHandler.java
@@ -17,7 +17,7 @@ import java.io.IOException;
 /**
  * Utility class that provides I/O operations for NFC tags.
  */
-public final class NfcTagHandler {
+public class NfcTagHandler {
     private final TagTechnology mTech;
     private final TagTechnologyHandler mTechHandler;
     private boolean mWasConnected;
@@ -49,7 +49,7 @@ public final class NfcTagHandler {
     private interface TagTechnologyHandler {
         public void write(NdefMessage message)
                 throws IOException, TagLostException, FormatException;
-        // TODO(crbug.com/625589): add read method for nfc.watch.
+        public NdefMessage read() throws IOException, TagLostException, FormatException;
     }
 
     /**
@@ -67,6 +67,10 @@ public final class NfcTagHandler {
                 throws IOException, TagLostException, FormatException {
             mNdef.writeNdefMessage(message);
         }
+
+        public NdefMessage read() throws IOException, TagLostException, FormatException {
+            return mNdef.getNdefMessage();
+        }
     }
 
     /**
@@ -84,9 +88,13 @@ public final class NfcTagHandler {
                 throws IOException, TagLostException, FormatException {
             mNdefFormattable.format(message);
         }
+
+        public NdefMessage read() throws IOException, TagLostException, FormatException {
+            return NfcTypeConverter.emptyNdefMessage();
+        }
     }
 
-    private NfcTagHandler(TagTechnology tech, TagTechnologyHandler handler) {
+    protected NfcTagHandler(TagTechnology tech, TagTechnologyHandler handler) {
         mTech = tech;
         mTechHandler = handler;
     }
@@ -102,6 +110,13 @@ public final class NfcTagHandler {
     }
 
     /**
+     * Checks if NFC tag is connected.
+     */
+    public boolean isConnected() {
+        return mTech.isConnected();
+    }
+
+    /**
      * Closes connection.
      */
     public void close() throws IOException {
@@ -115,6 +130,10 @@ public final class NfcTagHandler {
         mTechHandler.write(message);
     }
 
+    public NdefMessage read() throws IOException, TagLostException, FormatException {
+        return mTechHandler.read();
+    }
+
     /**
      * If tag was previously connected and subsequent connection to the same tag fails, consider
      * tag to be out of range.