Remove fixed layout methods from Blink

https://codereview.chromium.org/26546006/ removed the need
for tests to explicitly enable fixed layout mode and
https://codereview.chromium.org/35503005/ removed all
Chromium-side users. This patch removes leftover uses in
LayoutTests and removes the methods from the API.

The sole survivor is setFixedLayoutSize as WebView still
relies on it.

BUG=285397

Review URL: https://codereview.chromium.org/35923003

git-svn-id: svn://svn.chromium.org/blink/trunk@160384 bbb929c8-8fbe-4397-9dbb-9b2b20218538