Speculative fix to a crash caused by hitting the dangerous download i…

…nfobar.

There is no good explanation of the crash:
1. When clicking on a dangerous download, we set mPendingRequest and start
the infobar.
2. When user dismiss the infobar, we reset mPendingRequest.
3. If mPendingRequest is non null, a 2nd request will not trigger any
infobar.
So at anytime, there are at most 1 dangerous infobar.
And if mPendingRequest is non null, there should be an infobar showing,
vice versa.

So this a speculative fix to the crash from the crash stack we have
seen.

BUG=579707

Review URL: https://codereview.chromium.org/1614453003

Cr-Commit-Position: refs/heads/master@{#372153}

chrome/android/java/src/org/chromium/chrome/browser/download/ChromeDownloadDelegate.java

@@ -58,6 +58,7 @@ private class DangerousDownloadListener implements SimpleConfirmInfoBarBuilder.L
         @Override
         public void onInfoBarButtonClicked(boolean confirm) {
             assert mTab != null;
+            if (mPendingRequest == null) return;
             if (mPendingRequest.hasDownloadId()) {
                 nativeDangerousDownloadValidated(mTab, mPendingRequest.getDownloadId(), confirm);
                 if (confirm) {