Frame::eventHandler() should return a reference

This function never return null, so make it return a reference.

BUG=None

Review URL: https://codereview.chromium.org/31973002

git-svn-id: svn://svn.chromium.org/blink/trunk@160287 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Fix nth-last child style recalc

The logic that was there was just wrong. If we're looking at the 2nd
element from the end, nth-last should be 1, not the index of the last
element. There's room for optimization here, but in the meantime, it's
important to fix this regression.

BUG=308336

Review URL: https://codereview.chromium.org/32733003

git-svn-id: svn://svn.chromium.org/blink/trunk@160288 bbb929c8-8fbe-4397-9dbb-9b2b20218538