Fix NPE crash in repost form warning.

Repro steps:
1.) Turn on "Don't keep activities"
2.) Navigate to a test form site:
   https://www.cs.tut.fi/~jkorpela/forms/testing.html
3.) Submit POST form
4.) Refresh
5.) Hit the home button (causing the activity to be destroyed
    per #1

It crashes in the destruction path as the fragment manager has
been destroyed.

Add a safety check to prevent the crash.

BUG=598196

Review URL: https://codereview.chromium.org/1836053002

Cr-Commit-Position: refs/heads/master@{#383574}

Review URL: https://codereview.chromium.org/1837073002 .

Cr-Commit-Position: refs/branch-heads/2661@{#409}
Cr-Branched-From: ef6f6ae-refs/heads/master@{#378081}

chrome/android/java/src/org/chromium/chrome/browser/RepostFormWarningDialog.java

@@ -91,6 +91,12 @@ public void onClick(DialogInterface dialog, int id) {
         return dialog;
     }
 
+    @Override
+    public void dismiss() {
+        if (getFragmentManager() == null) return;
+        super.dismiss();
+    }
+
     @Override
     public void onDismiss(DialogInterface dialog) {
         super.onDismiss(dialog);