Make sure the Contextual Search supplier is non-null

After migrating contextual search scene-related logic out of
LayoutManager in http://crrev.com/c/2456634, it is possible for the
manager to be null. This patch adds a check for that case.

Bug: 1139765
Change-Id: I80c5c326a3a31dcfee378241f794c5860eac5370
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2484932Reviewed-by: Theresa  <twellington@chromium.org>
Commit-Queue: Matthew Jones <mdjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818484}

chrome/android/java/src/org/chromium/chrome/browser/ui/RootUiCoordinator.java

@@ -229,7 +229,9 @@ public class RootUiCoordinator
                 if (layout instanceof StaticLayout || layout instanceof SimpleAnimationLayout) {
                     return;
                 }
-                mContextualSearchManagerSupplier.get().dismissContextualSearchBar();
+                if (mContextualSearchManagerSupplier.get() != null) {
+                    mContextualSearchManagerSupplier.get().dismissContextualSearchBar();
+                }
             }
 
             @Override