Android: Fix a crash upon PIP mode exit

This CL caches the activity tab that PIP mode got turned on with,
to use it when leaving PIP mode. In this way, the reported NPE
can be avoided when the activity tab is not available at the time
callback needs to be invoked.

Bug: 1119340
Change-Id: I5d422b4d41961a09cc8c6c1f18ca89edf50d8eed
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2371768Reviewed-by: Peter Conn <peconn@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Commit-Queue: Jinsuk Kim <jinsukkim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806509}

chrome/android/java/src/org/chromium/chrome/browser/media/PictureInPictureController.java

@@ -200,17 +200,17 @@ public class PictureInPictureController {
         // We don't want InfoBars displaying while in PiP, they cover too much content.
         InfoBarContainer.get(activity.getActivityTab()).setHidden(true);
 
+        // Setup observers to dismiss the Activity on events that should end PiP.
+        final Tab activityTab = activity.getActivityTab();
+
         mOnLeavePipCallbacks.add(new Callback<ChromeActivity>() {
             @Override
             public void onResult(ChromeActivity activity2) {
                 webContents.setHasPersistentVideo(false);
-                InfoBarContainer.get(activity.getActivityTab()).setHidden(false);
+                InfoBarContainer.get(activityTab).setHidden(false);
             }
         });
 
-        // Setup observers to dismiss the Activity on events that should end PiP.
-        final Tab activityTab = activity.getActivityTab();
-
         final TabObserver tabObserver = new DismissActivityOnTabEventObserver(activity);
         final TabModelSelectorObserver tabModelSelectorObserver =
                 new DismissActivityOnTabModelSelectorEventObserver(activity);