Focus changes should be use affectedBy optimization

Previously any time you hide/showed your Chrome window
we would recalcStyle on the entire document (<body> and below).

Similarly whenever you clicked on anything in Chrome
we would recalc style of the entire body (unless the caret
was already somewhere else).  For applications which don't
normally take focus, this meant a zillion extra full-document
style recalcs.

This was especially bad for applications with animation triggered
from click, as those animations would first cause a full document
style recalc before their animation could start.

I don't believe we have any way to count style recalcs in a test
so there is no good way to test this yet.  I could write a
performance microbenchmark, but that would be more like a
regression test since it just tests for the presence of this
optimization.

BUG=315923

Review URL: https://codereview.chromium.org/63183004

git-svn-id: svn://svn.chromium.org/blink/trunk@161499 bbb929c8-8fbe-4397-9dbb-9b2b20218538