Add null check to fix crash in blink::ComputedStyle::display

These is a case that node->computedStyle() returns null
so it needs to check whether computedStyle() returns
null or not.

BUG=626991
TEST=editing/execCommand/crash-inserting-span.html

Review-Url: https://codereview.chromium.org/2135993003
Cr-Commit-Position: refs/heads/master@{#405733}
3 files changed