Avoid unnecessary style recalc for subtree of focused element.

Avoid unnecessary style recalc for subtree of focused element.

*** Relanding https://codereview.chromium.org/135183002/

Updating the styles for an element that changes its focus state only needs to
force a recalc of the subtree if the subtree contains an element that could
match a selector which contains a :focus pseudo class left of child or
descendant combinator. At the same time, that :focus pseudo must match the
element we are updating focus state for. This is indicated by the return value
of Element::childrenAffectedByFocus.

This change does a LocalStyleChange when updating the focus state of an
element which returns false for childrenAffectedByFocus, and
SubtreeStyleChange otherwise.

TBR=eseidel
BUG=315923
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=167598

[rune, 2014-02-21 13:38:01]

The CQ bit was checked by rune@opera.com

[commit-bot: I haz the power, 2014-02-21 13:38:10]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/rune@opera.com/169643003/1

[commit-bot: I haz the power, 2014-02-21 14:51:03]

Change committed as 167598