Fix an assertion failure in rendererIsFocusable of form controls.

If a form control has :active CSS rule, pressing a label element for the
control can make the control needsLayout state. We should make sure that
isMouseFocusable uses the latest layout information by calling
updateLayoutIgnorePendingStylesheets.

BUG=255316
TEST=automated

Review URL: https://chromiumcodereview.appspot.com/18216003

git-svn-id: svn://svn.chromium.org/blink/trunk@153369 bbb929c8-8fbe-4397-9dbb-9b2b20218538