RenderStyle(bool) is a footgun

C++ will happily convert a RenderStyle* to a RenderStyle using the Boolean
constructor. This CL both marks the constructor explicit and changes it to use
an explicit enum tag.

R=esprehn

Review URL: https://chromiumcodereview.appspot.com/23435007

git-svn-id: svn://svn.chromium.org/blink/trunk@157619 bbb929c8-8fbe-4397-9dbb-9b2b20218538
[CSS Grid Layout] Properly support for z-index on grid items

The specification says that grid should match flexbox and force
grid items to be stacking context if z-index is set, regardless
of 'position'. After this change, this is what happens.

BUG=234200

Review URL: https://chromiumcodereview.appspot.com/23600035

git-svn-id: svn://svn.chromium.org/blink/trunk@157620 bbb929c8-8fbe-4397-9dbb-9b2b20218538