RuleFetureSet should not reset StyleInvalidator.

When a new sheet is loaded before StyleInvalidator works, StyleInvalidator
misses style recalc for some elements, because elements' needsStyleInvalidation
flags are not cleared in StyleInvalidator::clearPendingInvalidations.

BUG=344729
TEST=fast/css/invalidation/sheet-loaded-before-invalidation.html

Review URL: https://codereview.chromium.org/225263016

git-svn-id: svn://svn.chromium.org/blink/trunk@171115 bbb929c8-8fbe-4397-9dbb-9b2b20218538