Implement a ':host' pseudo class

W3C bug:
https://www.w3.org/Bugs/Public/show_bug.cgi?id=22390

About :context behavior (:host is the same as :context. :context is renamed and now :host):
https://docs.google.com/a/google.com/document/d/1Az5JGK3EiBkH6zjx7U_RQrdTWYdC7vnK866Yt6dYTAs/edit

BUG=248011
TESTS=fast/dom/shadow/host-pseudo-class-css-text.html,fast/dom/shadow/host-pseudo-class.html
R=dglazkov@chromium.org

Review URL: https://codereview.chromium.org/19791005

git-svn-id: svn://svn.chromium.org/blink/trunk@155832 bbb929c8-8fbe-4397-9dbb-9b2b20218538
input[type=date] with invalid date should return empty string for value IDL attribute instead of the default value.

BaseDateAndTimeInputType::sanitizeValue should return an empty string if the
input string is invalid.

BUG=270310
R=keishi@chromium.org

Review URL: https://codereview.chromium.org/22727003

git-svn-id: svn://svn.chromium.org/blink/trunk@155833 bbb929c8-8fbe-4397-9dbb-9b2b20218538