[Forms] Empty visible value of AM/PM field of multiple fields time input UI should display variable number of "-" based on maximum number of labels
https://bugs.webkit.org/show_bug.cgi?id=95660

Reviewed by Kent Tamura.

This patch changes visible empty value of AM/PM field in multiple
fields time input UI to "-", "--", "---", and so on based on maximum
length of AM/PM labels. In current implementation, it is always "--"
regardless locale, although on Arabic, AM/PM are represented in one
character.

This patch affects ports which enable both ENABLE_INPUT_TYPE_TIME and
ENABLE_INPUT_TYPE_TIME_MULTIPLE_FIELDS.

No new tests. Once multiple field time input UI uses "lang" HTML
attribute to take time format, we can write a test for this patch.

* html/shadow/DateTimeSymbolicFieldElement.cpp:
(makeVisibleEmptyValue): Build visible empty value from maximum length
of labels.
(WebCore::DateTimeSymbolicFieldElement::visibleEmptyValue): Changed to
use m_visibleEmptyValue.
* html/shadow/DateTimeSymbolicFieldElement.h:
(WebCore::DateTimeSymbolicFieldElement): Added new member variable
m_visibleEmptyValue to hold visible empty value.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@127400 268f45cc-cd09-0410-ab3c-d52691b4dbfc