Adds CSS rules on the virtual keyboard conformant to native shadowdom.

BUG=352267

Review URL: https://codereview.chromium.org/199413002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@257107 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 257107