Reudce dependency of host element renderer type from text field shadow elements.

- Move RenderTextControlSingleLine::createInnerBlockStyle to
  TextControlInnerElement::customStyleForRenderer.
- Make TextControlInnerTextElement::customStyleForRenderer work with
  non-RenderTextControl.
- Remove unnecessary renderer access in SearchFieldDecorationElement::
  defaultEventHandler.

Note that we can't move RenderTextControlSingleLine::createInnerTextStyle
to TextControlInnerTextElement easily because the function accesses a
private data member of RenderTextControlSingleLine.

BUG=274205
TEST=none; This should not make any behavior changes.

Review URL: https://chromiumcodereview.appspot.com/23296005

git-svn-id: svn://svn.chromium.org/blink/trunk@156351 bbb929c8-8fbe-4397-9dbb-9b2b20218538