Mark NGBlockNode::FirstChild() as const member function

This patch marks |NGBlockNode::FirstChild()| as const member function since it
doesn't mutate its member for improving code health.

Note: This patch is a preparation for using |FirstChild()| in |CanNewLayout()|
for disable content editable in LayoutNG[1].

[1] http://crrev.com/c/792814  [LayoutNG] Disable LayoutNG for contenteditable

Bug: 591099
Change-Id: I205aea86bc15c47830c9c86e5949bd688c9c38e5
Reviewed-on: https://chromium-review.googlesource.com/792813
Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519976}
2 files changed