Refactoring: Remove searchDecorationElement() and cancelButtonElement() from HTMLInputElement and InputType.

Accessing these elements by ID names instead of holding pointers in
SearchInputType.
Move InputType::uaShadowElementById to Element because we'd like to use
it for UA shadows of other elements.

BUG=none

Review URL: https://chromiumcodereview.appspot.com/22733003

git-svn-id: svn://svn.chromium.org/blink/trunk@155918 bbb929c8-8fbe-4397-9dbb-9b2b20218538