Do not add features to classes in rightmost compound selector.

Features found in the rightmost compound selector should always have empty
invalidation sets. Their invalidation set should not contain itself or any
other features found in the same compound selector.

No need to look for descendants with classes 'a' or 'b' when changing 'a'
or 'b' on elements for the following selectors:

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

R=esprehn@chromium.org, chrishtr@chromium.org
BUG=345238

Review URL: https://codereview.chromium.org/171513013

git-svn-id: svn://svn.chromium.org/blink/trunk@167674 bbb929c8-8fbe-4397-9dbb-9b2b20218538