[selectors4] Use common ancestor strategy for :focus-within
Following the same approach than what we do for :active and :hover
pseudo-classes, it seems a nice optimization to use the common ancestor
strategy for :focus-within too.
The use case would be for example that you've a from with several inputs
in a deep part of the DOM, and in the body element you've :focus-within
to show some warning while editing the form.
When you switch focus from one input to the next one, we don't need
to invalidate the whole chain up to the body, but it'd be enough
to just do it up to the common ancestor of the old and new focused elements
(in this example the form).
To achieve that Document::SetFocusedElement() has been modified to look
for the common ancestor and then pass it to
ContainerNode::SetHasFocusWithinUpToAncestor().
This new helper method is also used from FocusController to set
:focus-within flag when the window loses/gains focus.
A new unit test has been added to AffectedByFocusTest.cpp
verifying that we only recalculate styles up to the common ancestor.
BUG=617371
TEST=fast/selectors/focus-within-window-inactive.html
Review-Url: https://codereview.chromium.org/2821303005
Cr-Commit-Position: refs/heads/master@{#466949}
5 files changed
