<webview> Add missing null check in FocusOwningWebContents.

Fix for https://crbug.com/666833

Browser crashed when trying to move focus out of a webview that has
crashed.

BUG=666833

Review-Url: https://codereview.chromium.org/2514963002
Cr-Commit-Position: refs/heads/master@{#433352}
2 files changed