Removed incorrect StyleInvalidator FIXME.

Cloning the RenderStyle, like the FIXME said, is not enough. It would be
true if we only scheduled invalidation sets for classes/ids/... not in the
rightmost compound selector, but we schedule invalidation sets for classes
on the element it needs to invalidate too. Setting class=a on an element
with the style rules below requires us to do a LocalStyleChange on both
'.a' and '.b' in order to apply the first rule as well. For setting
class=x, that wouldn't be necesarry.

.a {}
.a .b {}
.x .y {}

It's possible to optimize that case by adding a flag to the invalidation
set to indicate that the feature the invalidation set is for is present in
the rightmost compound of some selector.

R=chrishtr@chromium.org,esprehn@chromium.org

Review URL: https://codereview.chromium.org/679663002

git-svn-id: svn://svn.chromium.org/blink/trunk@184394 bbb929c8-8fbe-4397-9dbb-9b2b20218538