Gracefully handle a null ChromeActivity when calling isUVPAA.

Bug: 913881
Change-Id: I291167a9f707afe19709aabf212e01aed8d4e201
Reviewed-on: https://chromium-review.googlesource.com/c/1372911Reviewed-by: Wei-Yin Chen (陳威尹) <wychen@chromium.org>
Commit-Queue: Kim Paulhamus <kpaulhamus@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615981}

chrome/android/java/src/org/chromium/chrome/browser/webauth/AuthenticatorImpl.java

@@ -95,6 +95,11 @@ public class AuthenticatorImpl implements Authenticator, HandlerResponseCallback
     public void isUserVerifyingPlatformAuthenticatorAvailable(
             IsUserVerifyingPlatformAuthenticatorAvailableResponse callback) {
         Context context = ChromeActivity.fromWebContents(mWebContents);
+        // ChromeActivity could be null.
+        if (context == null) {
+            callback.call(false);
+        }
+
         if (PackageUtils.getPackageVersion(context, GMSCORE_PACKAGE_NAME) < GMSCORE_MIN_VERSION
                 || Build.VERSION.SDK_INT < Build.VERSION_CODES.N) {
             callback.call(false);