Reland: Prepare for multiple !important author ranges.

Prepare for multiple !important author ranges.

This CL is split out from [1] where we support multiple author origins in
terms of shadow dom scopes. The vector of MatchedProperties contains
matched declarations from all origins in the order in which they should
be applied. For !important declarations, they are applied in the same
order within the same origin, but the order of the origins are different
as the cascading order is:

  - UA important
  - Author important
  - Author
  - UA

We used to have two ranges, one for UA and one for author to accomplish
this. However, with the normal/!important cascading order for
declarations from different shadow doms, we need multiple author ranges:

  - UA important
  - Author important scope n
  ...
  - Author important scope 1
  - Author scope 1
  ...
  - Author scope n
  - UA

We introduce an ImportantAuthorRangeIterator to iterate through the
author ranges and add iterator-style iteration for each range through the
MatchedPropertiesRange class.

As the applyMatchedProperties code is hot, I've run the blink_perf.css
tests locally. Some improved by 2-4%, some became worse by similar
percentages.

[1] https://codereview.chromium.org/1224673002/

BUG=487125
TEST=fast/css

Reland of: https://codereview.chromium.org/1282243002 with Oilpan fixes.

[rune, 2015-08-19 08:37:30]

The CQ bit was checked by rune@opera.com to run a CQ dry run

[commit-bot: I haz the power, 2015-08-19 08:37:44]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1283133006/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1283133006/20001