Fix NPE during NewTabPageLayout destruction.

Null pointer crash is caused by CompositorViewHolder nulled out before onDestroy gets called in some cases.

Bug: 904245
Change-Id: I4a8c3347a2bf7f0af2f704bc2e33cadc4534adc7
Reviewed-on: https://chromium-review.googlesource.com/c/1334273
Commit-Queue: Cathy Li <chili@chromium.org>
Reviewed-by: Theresa <twellington@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607752}
1 file changed