Fix assert in CustomTabActivityTabProvider#swapTab()

This CL moves the assert after the identity check so that calling
swapTab(null) after removeTab() does not cause the assert to fail.

BUG=1083102

Change-Id: Ib11776451f5b9fef60a9be5e0ee5bd7efc7b790b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2202695Reviewed-by: Peter Conn <peconn@chromium.org>
Commit-Queue: Peter Kotwicz <pkotwicz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#770408}

chrome/android/java/src/org/chromium/chrome/browser/customtabs/content/CustomTabActivityTabProvider.java

@@ -108,8 +108,8 @@ public class CustomTabActivityTabProvider {
     }
 
     public void swapTab(@Nullable Tab tab) {
-        assert mTab != null : "swapTab shouldn't be called before setInitialTab";
         if (mTab == tab) return;
+        assert mTab != null : "swapTab shouldn't be called before setInitialTab";
         mTab = tab;
         if (mTab == null) {
             for (Observer observer : mObservers) {