Fix tab leak in BottomSheetController

Previously the BottomSheetController held on to the last activity tab
until a new tab was selected. This caused a memory leak if all tabs
were closed. This patch clears the last active tab if that tab is
destroyed.

Bug: 891094
Change-Id: I84343727dba0719792e115675153c8fffba5346d
Reviewed-on: https://chromium-review.googlesource.com/1256812Reviewed-by: Theresa <twellington@chromium.org>
Commit-Queue: Matthew Jones <mdjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595952}

chrome/android/java/src/org/chromium/chrome/browser/widget/bottomsheet/BottomSheetController.java

@@ -113,6 +113,11 @@ public class BottomSheetController implements ApplicationStatus.ActivityStateLis
             public void onCrash(Tab tab, boolean sadTabShown) {
                 clearRequestsAndHide();
             }
+
+            @Override
+            public void onDestroyed(Tab tab) {
+                if (mLastActivityTab == tab) mLastActivityTab = null;
+            }
         };
 
         mTabProvider.addObserverAndTrigger(new HintlessActivityTabObserver() {