This makes it possible to specify and lock the fixedLayoutSize.

The reason for this change is android_webview/ wrap_content mode where the size of
the WebView depends on the size of its contents. Without forcing the layout size
to a fixed value, such a configuration becomes unstable when elements whose size is
relative to the viewport size are present in the contents.

This also adds support for zero-height, non-zero width layout sizes.

BUG=246621

Review URL: https://chromiumcodereview.appspot.com/23441020

git-svn-id: svn://svn.chromium.org/blink/trunk@157749 bbb929c8-8fbe-4397-9dbb-9b2b20218538