Make isInLayout more specific.

The m_inLayout flag is misleading as it doesn't tell you if you're in layout
but if you're currently doing the performLayout() work. This Cl renames the
flag to be more specific.

The current code allows you to enter ::layout() recursively, as long as you
aren't currently doing the performLayout() step. (This can be see in the
use of the m_nestedLayoutCounter in the code.)

BUG=

Review URL: https://codereview.chromium.org/137133004

git-svn-id: svn://svn.chromium.org/blink/trunk@165667 bbb929c8-8fbe-4397-9dbb-9b2b20218538