BUILD FIX (r154778): AXSearchFieldCancelButtonText() is not defined on iOS

BUILD FIX (r154778): AXSearchFieldCancelButtonText() is not defined on iOS

See: AX: Cancel button in search field not accessible.
<https://webkit.org/b/120322>

Fixes the following build error:

    In file included from Source/WebCore/accessibility/AccessibilityAllInOne.cpp:44:
    Source/WebCore/accessibility/AccessibilitySearchFieldButtons.cpp:46:12: error: use of undeclared identifier 'AXSearchFieldCancelButtonText'
        return AXSearchFieldCancelButtonText();
               ^
    1 error generated.

* accessibility/AccessibilitySearchFieldButtons.cpp:
(WebCore::AccessibilitySearchFieldCancelButton::accessibilityDescription):
Return String() instead of calling AXSearchFieldCancelButtonText() on iOS.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@154812 268f45cc-cd09-0410-ab3c-d52691b4dbfc