Fix TabGroup crashing when closing all tabs

When "close all tabs", the current tab which is the next tab can be
null. We should use the tab from parameter instead of using current tab.

Change-Id: Ic696b68131755e0b4224e1263995340a4e30b178
Bug: 968949
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1639009
Commit-Queue: Mei Liang <meiliang@chromium.org>
Reviewed-by: Wei-Yin Chen (陳威尹) <wychen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665415}

chrome/android/features/tab_ui/java/src/org/chromium/chrome/browser/tasks/tab_management/TabGroupUiMediator.java

@@ -97,14 +97,11 @@ public class TabGroupUiMediator {
             @Override
             public void willCloseTab(Tab tab, boolean animate) {
                 if (!mIsTabGroupUiVisible) return;
-                Tab currentTab = mTabModelSelector.getCurrentTab();
-                if (currentTab == null) mResetHandler.resetSheetWithListOfTabs(null);
-                int tabsCount = mTabModelSelector.getTabModelFilterProvider()
-                                        .getCurrentTabModelFilter()
-                                        .getRelatedTabList(currentTab.getId())
-                                        .size();
+                List<Tab> group = mTabModelSelector.getTabModelFilterProvider()
+                                          .getCurrentTabModelFilter()
+                                          .getRelatedTabList(tab.getId());
 
-                mIsClosingAGroup = tabsCount == 0;
+                mIsClosingAGroup = group.size() == 0;
             }
 
             @Override