https://bugs.webkit.org/show_bug.cgi?id=121058

Reviewed by Kent Tamura.

Source/WebCore: 

HTMLTextFormControl::fixPlaceholderRenderer was added in r118733 to swap the order in which placeholder appears.
Namely, when a text form control element is focused, placeholder should be rendered behind the text for the caret
to render on top of, not behind, the placeholder text.  However, we can achieve the same effect by changing
the order of elements in the shadow DOM instead of manually manipuating the render tree as the assertion failure
mentioned in the change log is a bogus one.

Cleaned up the code by removing HTMLTextFormControl::fixPlaceholderRenderer and changed the order in which inner
text element and placeholder element appear. Also fixed the assertion in Position's constructor.

* dom/Position.cpp:
(WebCore::Position::Position): The original assertion was bogus. What we don't want have is for a Position to be
before or after a shadow root. It's fine for it be anchroed against the shadow root as long as it's inside.

* html/HTMLTextAreaElement.cpp:
(WebCore::HTMLTextAreaElement::updatePlaceholderText): Insert the placeholder before the inner text element.
* html/HTMLTextAreaElement.h:
* html/HTMLTextFormControlElement.cpp:
* html/HTMLTextFormControlElement.h:
* html/TextFieldInputType.cpp:
(WebCore::TextFieldInputType::updatePlaceholderText): Ditto.
* html/TextFieldInputType.h:

LayoutTests: 

Rebaseline tests now that the placeholder appears before the inner text element.

* fast/forms/placeholder-position-expected.txt:
* platform/mac/fast/forms/input-placeholder-visibility-1-expected.txt:
* platform/mac/fast/forms/input-placeholder-visibility-3-expected.txt:
* platform/mac/fast/forms/textarea-placeholder-pseudo-style-expected.txt:
* platform/mac/fast/forms/textarea-placeholder-visibility-1-expected.txt:
* platform/mac/fast/forms/textarea-placeholder-visibility-2-expected.txt:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@155408 268f45cc-cd09-0410-ab3c-d52691b4dbfc