Add ease-in animation for show and hide keyboard

BUG=255161
R=jamescook@chromium.org, sadrul@chromium.org

Review URL: https://codereview.chromium.org/102653002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239495 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239495