Preserve find bar selection across tab switches.

This also removes SearchTextfieldView from the Windows code, which did nothing
the underlying Textfield wasn't already doing.

BUG=297385
TEST=Open the find bar in two tabs, select different text in each find bar, and toggle between the two tabs.  The selections should be preserved.
R=finnur@chromium.org, sail@chromium.org

Review URL: https://codereview.chromium.org/24449008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226039 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226039