Android: Fix Favicon NPE in accessibility mode

TabFavicon is Tab's UserData which should not be accessed after
Tab is destroyed or its WebContent is not ready yet. This CL adds
the corresponding check to avoid NPE that occurs when working
in accessibility mode, which attempts to access a Tab already
destroyed after an animation effect or Tab is not loaded with
a new WebContents object.

Bug: 940332
Change-Id: If77c0eab251fb0bc728f046605a9f6930187557b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1516116
Reviewed-by: Theresa <[email protected]>
Commit-Queue: Jinsuk Kim <[email protected]>
Cr-Original-Commit-Position: refs/heads/master@{#639735}(cherry picked from commit 01084125cf95d52cf9b06fafcc2eac07098302de)
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1518841
Reviewed-by: Krishna Govind <[email protected]>
Cr-Commit-Position: refs/branch-heads/3729@{#56}
Cr-Branched-From: d4a8972-refs/heads/master@{#638880}

Author: JinsukKim

chrome/android/java/src/org/chromium/chrome/browser/tab/TabFavicon.java

@@ -41,6 +41,7 @@ static TabFavicon from(Tab tab) {
     }
 
     private static TabFavicon get(Tab tab) {
+        if (tab == null || !tab.isInitialized()) return null;
         return tab.getUserDataHost().getUserData(USER_DATA_KEY);
     }