Remove out-of-date FIXME on collapsing margins

CSS2.1 is now clear about what to do if the collapsed margins are
greater than the offset required to clear a float.

"Computing the clearance of an element on which 'clear' is set is done
by first determining the hypothetical position of the element's top
border edge. This position is where the actual top border edge would
have been if the element's 'clear' property had been 'none'. If this
hypothetical position of the element's top border edge is not past
the relevant floats, then clearance [..] is set to the greater of:
The amount necessary to place the border edge of the block even with
the bottom outer edge of the lowest float that is to be cleared. [or]
The amount necessary to place the top border edge of the block at
its hypothetical position."

http://www.w3.org/TR/CSS2/visuren.html#clearance


BUG=

Review URL: https://codereview.chromium.org/195743002

git-svn-id: svn://svn.chromium.org/blink/trunk@169798 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Only :host and :ancestor should ever match the host in a ShadowRoot

We were blocking tag, id and class from matching when put before :host, but were
not correctly stopping things like :first-child:host or *:host.

Instead of trying to black list selectors we need to whitelist :host and
:ancestor as being special.

It turns out we did have test coverage for this, but the expected results were
all wrong so it printed PASS even though we were failing what the test was
supposed to be testing.

BUG=355149, 355158

Review URL: https://codereview.chromium.org/208933006

git-svn-id: svn://svn.chromium.org/blink/trunk@169799 bbb929c8-8fbe-4397-9dbb-9b2b20218538