Do not construct and destruct new InputType instance if it is not used.

Fix a bug that UI elements in multiple-fields inputs don't work after
input.type = input.type.
This bug is a regression by Blink r151841 [1]

BaseMultipleFieldsDateAndTimeInputType destructor disconnects UI elments from
their owners. Constructing and destructing unused
BaseMultipleFieldDateAndTimeInputType object in HTMLInputType::updateType
diconnected existing ownership relations build by another
BaseMultipleFieldDateAndTimeInputType object.

[1] http://src.chromium.org/viewvc/blink?revision=151841&view=revision

BUG=277843
TEST=automated

Review URL: https://chromiumcodereview.appspot.com/22796031

git-svn-id: svn://svn.chromium.org/blink/trunk@156622 bbb929c8-8fbe-4397-9dbb-9b2b20218538