[selectors4] Implement :focus-within pseudo-class
This patch adds support for the new ":focus-within" pseudo-class.
Most of the patch is basically the regular boilerplate code
to add a new selector. Then the interesting changes happen
on ContainerNode.
The patch is covered by a bunch of tests imported from
the W3C test suite (which includes generic tests in addition to
the tests from Firefox and WebKit implementations too).
Apart from some minor changes on current tests to add
the new selector.
Intent-to-ship thread is available at:
https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/V3RNBhQelSg
BUG=617371
Review-Url: https://codereview.chromium.org/2795143004
Cr-Commit-Position: refs/heads/master@{#463992}
25 files changed
