[webnfc] Do not disable NFC reader mode if nfc is turned off

Don't disable NFC reader mode when feature is turned off, since
the system NFC manager returns null NFCAdapter.

BUG=640890,640995

Review-Url: https://codereview.chromium.org/2315203002
Cr-Commit-Position: refs/heads/master@{#417003}

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

@@ -346,7 +346,7 @@ public class NfcImpl implements Nfc {
     private void enableReaderMode() {
         if (Build.VERSION.SDK_INT < Build.VERSION_CODES.KITKAT) return;
 
-        if (mReaderCallbackHandler != null || mActivity == null) return;
+        if (mReaderCallbackHandler != null || mActivity == null || mNfcAdapter == null) return;
 
         // TODO(crbug.com/625589): Check if there are active watch operations.
         if (mPendingPushOperation == null) return;
@@ -366,8 +366,12 @@ public class NfcImpl implements Nfc {
     private void disableReaderMode() {
         if (Build.VERSION.SDK_INT < Build.VERSION_CODES.KITKAT) return;
 
+        // There is no API that could query whether reader mode is enabled for adapter.
+        // If mReaderCallbackHandler is null, reader mode is not enabled.
+        if (mReaderCallbackHandler == null) return;
+
         mReaderCallbackHandler = null;
-        if (mActivity != null && !mActivity.isDestroyed()) {
+        if (mActivity != null && mNfcAdapter != null && !mActivity.isDestroyed()) {
             mNfcAdapter.disableReaderMode(mActivity);
         }
     }