Yosemite: Don't show the default placeholder of NSSearchFieldCell.

We need to call setCenteredLook:NO on Yosemite.
This CL was inspired by WebKit r168870 by dino@apple [1].

[1] http://trac.webkit.org/changeset/168870

BUG=394714
TEST=None for now; This change will be covered with existing tests
    when we add Yosemite buildbot.

Review URL: https://codereview.chromium.org/586413002

git-svn-id: svn://svn.chromium.org/blink/trunk@182445 bbb929c8-8fbe-4397-9dbb-9b2b20218538