[Android] Fix sign-in crash fix.

Sign-in button on sign-in promotion panel can be clicked multiple times,
which can lead to a crash. We should make sure that signing in logic only
happens once.

BUG=477139

Review URL: https://codereview.chromium.org/1073943003

Cr-Commit-Position: refs/heads/master@{#327200}

chrome/android/java/src/org/chromium/chrome/browser/signin/SigninManager.java

@@ -280,9 +280,10 @@ public class SigninManager {
      */
     public void startSignIn(Activity activity, final Account account, boolean passive,
             final SignInFlowObserver observer) {
-        assert mSignInActivity == null;
-        assert mSignInAccount == null;
-        assert mSignInFlowObserver == null;
+        if (mSignInAccount != null) {
+            Log.w(TAG, "Ignoring sign-in request as another sign-in request is pending.");
+            return;
+        }
 
         if (mFirstRunCheckIsPending) {
             Log.w(TAG, "Ignoring sign-in request until the First Run check completes.");