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This CL implements the shadow tree cascade order proposed in [1].

Previously, in Blink, specificity would win over scope origin, even if the
scopes had an inner/outer scope relationship, and the order-of-appearance
was governed by the CascadeOrder type. Also, !important rules did not
apply in the reverse scope order, as the current spec says for inner/outer
scopes, and the proposal in [1] says apply between all shadow scopes.

What has been done is:

1. CascadeOrder is not used, as it represents order-of-appearance
   (Removal of CascadeOrder is not done here to make the CL smaller. Will
   be removed in a follow-up CL).

2. When collecting rules, sort after each scope instead of just after UA
   and Author since we had:

   UA important
   Author important
   Author
   UA

   and now we have (with A(n) appearing before A(n+1) in the tree-of-trees
   order):

   UA important
   Author scope A(n) important
   ...
   Author scope A(1) important
   Author scope A(1)
   ...
   Author scope A(n)
   UA

   The applyProperties code is hot, and I have made performance runs for
   the micro-benchmarks in Layout and CSS without consistent regressions.

3. Since the cascading order between scopes are just the inner/outer
   relationship in the composed tree (direction decided by !important),
   which is the same as the tree-of-trees order of the shadow trees,
   we can just traverse the DocumentOrderedList of scopes in the reverse
   order instead of doing calculation tricks for CascadeOrder values.

   Because of this, TreeBoundaryCrossingRules is now reduced to a
   DocumentOrderedList of scoping nodes, so the TreeBoundaryCrossingRules
   class is removed.

[1] https://lists.w3.org/Archives/Public/www-style/2015Jun/0303.html

BUG=452542, 455148, 487125

NOTRY=true

Review URL: https://codereview.chromium.org/1298173004

git-svn-id: svn://svn.chromium.org/blink/trunk@200994 bbb929c8-8fbe-4397-9dbb-9b2b20218538