Access placeholder shadow element with ID.

It's safer than holding pointers in the host element. We reduce code size and
memory consumption.

BUG=none
TEST=none; no behavior changes.

Review URL: https://chromiumcodereview.appspot.com/23570012

git-svn-id: svn://svn.chromium.org/blink/trunk@158005 bbb929c8-8fbe-4397-9dbb-9b2b20218538
[Font Load Events] Implement FontFaceSet methods

* A new method match() returns font faces that match a given font string
* checkFont() is renamed to check()
* loadFont() is translated to load(), which returns Promise that fulfills
  when all loads complete
* notifyWhenFontsReady() is translated to ready(), which returns Promise
* Boolean 'loading' attribute is changed to string 'status' attribute

spec: http://dev.w3.org/csswg/css-font-load-events/#FontFaceSet-interface

BUG=53213
TEST=fast/css

Review URL: https://chromiumcodereview.appspot.com/23717059

git-svn-id: svn://svn.chromium.org/blink/trunk@158006 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Update tests for crrev.com/223985, Update validation messsage strings for stepMismatch.

BUG=161733
TBR=keishi@chromium.org

Review URL: https://codereview.chromium.org/23531067

git-svn-id: svn://svn.chromium.org/blink/trunk@158008 bbb929c8-8fbe-4397-9dbb-9b2b20218538