Fix SyncErrorInfoBar for null WebContent

Crashes in chrome canary might be because SyncErrorInfoBar is passed null
WebContent on launch from the active tab.

Bug: 1048300
Change-Id: I2042b020e9b82c4fc10b876a3b83aed1de22fb57
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2035986Reviewed-by: Boris Sazonov <bsazonov@chromium.org>
Commit-Queue: Tanmoy Mollik <triploblastic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#738517}

chrome/android/java/src/org/chromium/chrome/browser/infobar/SyncErrorInfoBar.java

@@ -122,7 +122,8 @@ public class SyncErrorInfoBar extends ConfirmInfoBar {
      * Calls native side code to create an infobar.
      */
     public static void maybeLaunchSyncErrorInfoBar(WebContents webContents) {
-        if (!ChromeFeatureList.isEnabled(ChromeFeatureList.SYNC_ERROR_INFOBAR_ANDROID)) {
+        if (!ChromeFeatureList.isEnabled(ChromeFeatureList.SYNC_ERROR_INFOBAR_ANDROID)
+                || webContents == null) {
             return;
         }
         @SyncErrorInfoBarType