Use static_cast instead of reinterpret_cast for downcasting

static_cast is better suited for downcasting since it can check that the
types are related.

/chrome/browser/ui/app_list/arc
This CL was uploaded by git cl split.

R=lgcheng@google.com

Change-Id: I3a6915dc3a04977b760d9555352831f81a7081a8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2435351
Auto-Submit: Kevin McNee <mcnee@chromium.org>
Reviewed-by: Long Cheng <lgcheng@google.com>
Commit-Queue: Long Cheng <lgcheng@google.com>
Cr-Commit-Position: refs/heads/master@{#811327}

chrome/browser/ui/app_list/arc/arc_app_unittest.cc

@@ -534,7 +534,7 @@ class ArcAppModelBuilderTest
       ChromeAppListItem* item = model_updater_->ItemAtForTest(i);
       if (item->GetItemType() == AppServiceAppItem::kItemType) {
         if (arc_count++ == index) {
-          arc_item = reinterpret_cast<AppServiceAppItem*>(item);
+          arc_item = static_cast<AppServiceAppItem*>(item);
           break;
         }
       }