Rename m_layoutRoot to m_layoutSubtreeRoot.

The layoutRoot is set when we're doing a layout of a subtree. Rename the
variable to make it more explicit that it points to a subtree.

This also adds is isASubtreeLayout() method to FrameView to replace the various
places we use if (m_layoutRoot) with if (isASubtreeLayout()) to make things
a bit clearer.

BUG=

Review URL: https://codereview.chromium.org/148023003

git-svn-id: svn://svn.chromium.org/blink/trunk@165841 bbb929c8-8fbe-4397-9dbb-9b2b20218538