Reinforce destruction path in awcontentsclientbridge.

Do not try to access native objects without making sure they are available.

BUG=

Review URL: https://codereview.chromium.org/267963002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@268031 0039d316-1c4b-4281-b951-d872f2087c98

android_webview/java/src/org/chromium/android_webview/AwContentsClientBridge.java

@@ -147,6 +147,7 @@ public class AwContentsClientBridge {
         }
 
         private void provideResponse(AndroidPrivateKey androidKey, byte[][] certChain) {
+            if (mNativeContentsClientBridge == 0) return;
             nativeProvideClientCertificateResponse(mNativeContentsClientBridge, mId,
                     certChain, androidKey);
         }
@@ -197,6 +198,7 @@ public class AwContentsClientBridge {
     @CalledByNative
     protected void selectClientCertificate(final int id, final String[] keyTypes,
             byte[][] encodedPrincipals, final String host, final int port) {
+        assert mNativeContentsClientBridge != 0;
         ClientCertLookupTable.Cert cert = mLookupTable.getCertData(host, port);
         if (mLookupTable.isDenied(host, port)) {
             nativeProvideClientCertificateResponse(mNativeContentsClientBridge, id,