[FastTextAutosizer] Check for containingBlock in deepestBlockContainingAllText

This patch adds another speculative assert in deepestBlockContainingAllText
to check for containingBlock() returning null which it can when the
subtree is detached.

I wasn't able to hit this in a synthetic test so this patch is a
speculative fix. If this improves stability we can consider guarding
all of the containingBlock() calls.

BUG=370764

Review URL: https://codereview.chromium.org/273613002

git-svn-id: svn://svn.chromium.org/blink/trunk@173581 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Auto-rebaseline for r173562

http://src.chromium.org/viewvc/blink?view=revision&revision=173562

BUG=361729
TBR=enne@chromium.org

Review URL: https://codereview.chromium.org/268363015

git-svn-id: svn://svn.chromium.org/blink/trunk@173582 bbb929c8-8fbe-4397-9dbb-9b2b20218538