Fix for https://bugs.webkit.org/show_bug.cgi?id=53740

Fix for https://bugs.webkit.org/show_bug.cgi?id=53740
<rdar://problem/8503629> Allow platforms to specify if the placeholder should be visible when text controls are focused

Reviewed by Dan Bernstein.

Tests:
fast/forms/textarea-placeholder-visibility-1.html
fast/forms/textarea-placeholder-visibility-2.html
fast/forms/input-placeholder-visibility-1.html
fast/forms/input-placeholder-visibility-2.html
fast/forms/input-placeholder-visibility-3.html

* html/HTMLFormControlElement.cpp: (WebCore::HTMLTextFormControlElement::placeholderShouldBeVisible):
Add a check for shouldShowPlaceholderWhenFocused.
* html/HTMLTextAreaElement.cpp: (WebCore::HTMLTextAreaElement::updateValue):
Whenever the value is updated, we should also update placeholder visibility.
* rendering/RenderTheme.h: (WebCore::RenderTheme::shouldShowPlaceholderWhenFocused):
Make the default the same as the existing behavior.
* rendering/RenderThemeMac.h:
* rendering/RenderThemeMac.mm: (WebCore::RenderThemeMac::shouldShowPlaceholderWhenFocused):
Show placeholder when appropriate.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@77639 268f45cc-cd09-0410-ab3c-d52691b4dbfc