Convert USERSELECT_ALL to a runtime enabled feature.

We'd like to turn this on again once yahoo mail fixes their site.
In the meantime, it can be runtime enabled so we keep it compiling
at least.

Review URL: https://chromiumcodereview.appspot.com/22751005

git-svn-id: svn://svn.chromium.org/blink/trunk@155893 bbb929c8-8fbe-4397-9dbb-9b2b20218538