[VK] Move ShouldWindowOverscroll to be private on ChromeKeyboardUI.

The base class KeyboardUI doesn't call ShouldWindowOverscroll, so it
doesn't really need to be on the base class. ShouldWindowOverscroll
is only used by ChromeKeyboardUI, so we just move it to be a private
method on that class.

TBR=yhanada@chromium.org

Change-Id: I5b95c83d64f6e2d8d2c8d50ac42bb34416a06aaa
Reviewed-on: https://chromium-review.googlesource.com/1150014
Reviewed-by: Darren Shen <shend@chromium.org>
Commit-Queue: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578137}
6 files changed