AX: Expose block quote level to iOS API

AX: Expose block quote level to iOS API
https://bugs.webkit.org/show_bug.cgi?id=166629
<rdar://problem/29834793>

Reviewed by Darin Adler.

Source/WebCore:

Expose the blockquote level to iOS Accessbility API.
Change the return type of blockquote level to unsigned.

Test: accessibility/ios-simulator/blockquote-level.html

* accessibility/AccessibilityObject.cpp:
(WebCore::AccessibilityObject::blockquoteLevel):
* accessibility/AccessibilityObject.h:
* accessibility/ios/WebAccessibilityObjectWrapperIOS.mm:
(-[WebAccessibilityObjectWrapper accessibilityBlockquoteLevel]):
* accessibility/mac/WebAccessibilityObjectWrapperMac.mm:
(-[WebAccessibilityObjectWrapper accessibilityAttributeValue:]):

Tools:

* DumpRenderTree/ios/AccessibilityUIElementIOS.mm:
(AccessibilityUIElement::numberAttributeValue):
* WebKitTestRunner/InjectedBundle/ios/AccessibilityUIElementIOS.mm:
(WTR::AccessibilityUIElement::numberAttributeValue):

LayoutTests:

* accessibility/ios-simulator/blockquote-level-expected.txt: Added.
* accessibility/ios-simulator/blockquote-level.html: Added.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@210222 268f45cc-cd09-0410-ab3c-d52691b4dbfc