Check isText() when editing traverses layout tree for ::first-letter

      This patch checks isText() before toLayoutText() when editing traverses
layout trees for ::first-letter pseudo elements.

Normally, the first child of ::first-letter pseudo element is
LayoutText. This patch checks if the assumption stands.

BUG=684684

Review-Url: https://codereview.chromium.org/2650953004
Cr-Commit-Position: refs/heads/master@{#445986}