[Cleanup] Downgrade CHECK to DCHECK in TabStripModel

This CHECK was added to (help) ensure we tracked down the cases when
we were trying to set an invalid opener. It's been a few cycles, and we
haven't seen any crashes, so downgrade to a DCHECK.

Bug: 700518
Change-Id: I3f607fe4ae7b98bbaac30fa910c0bf7394388270
Reviewed-on: https://chromium-review.googlesource.com/693309Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#506611}

chrome/browser/ui/tabs/tab_strip_model.cc

@@ -555,16 +555,11 @@ WebContents* TabStripModel::GetOpenerOfWebContentsAt(int index) {
 void TabStripModel::SetOpenerOfWebContentsAt(int index,
                                              WebContents* opener) {
   DCHECK(ContainsIndex(index));
-  if (opener) {
-    // The TabStripModel only maintains the references to openers that it itself
-    // owns; trying to set an opener to an external WebContents can result in
-    // the opener being used after its freed. See crbug.com/698681.
-    // TODO(devlin): This is a CHECK right now to track down any other cases
-    // where this can happen. If there aren't any, we might be able to downgrade
-    // this to a DCHECK.
-    CHECK_NE(kNoTab, GetIndexOfWebContents(opener))
-        << "Cannot set opener to a web contents not owned by this tab strip.";
-  }
+  // The TabStripModel only maintains the references to openers that it itself
+  // owns; trying to set an opener to an external WebContents can result in
+  // the opener being used after its freed. See crbug.com/698681.
+  DCHECK(!opener || GetIndexOfWebContents(opener) != kNoTab)
+      << "Cannot set opener to a web contents not owned by this tab strip.";
   contents_data_[index]->set_opener(opener);
 }