Use tabContentManagerSupplier which is guaranteed to be non-null.

Bug: 1126823
Change-Id: I32ec909f844c29433d1b60e0048ba725052718f4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2475575Reviewed-by: Matthew Jones <mdjones@chromium.org>
Reviewed-by: Wei-Yin Chen (陳威尹) <wychen@chromium.org>
Commit-Queue: Hao Dong <spdonghao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#817966}

chrome/android/java/src/org/chromium/chrome/browser/compositor/layouts/LayoutManagerChrome.java

@@ -105,7 +105,7 @@ public class LayoutManagerChrome
                 if (mOverviewLayout != null) {
                     mOverviewLayout.setTabContentManager(manager);
                 }
-                mTabContentManagerSupplier.removeObserver(this);
+                tabContentManagerSupplier.removeObserver(this);
             }
         });