AX: when invert colors is on, double-invert video elements in UserAgentStyleSheet

AX: when invert colors is on, double-invert video elements in UserAgentStyleSheet
https://bugs.webkit.org/show_bug.cgi?id=168447
<rdar://problem/30559874>

Reviewed by Simon Fraser.

Double-invert video when platform "invert colors" setting is enabled. Behavior matches
current "Smart Invert" feature of Safari Reader on macOS/iOS and other iOS native apps.

Source/WebCore:

Tests: accessibility/smart-invert-reference.html
       accessibility/smart-invert.html

* Modules/modern-media-controls/controls/media-controls.css:
(@media (inverted-colors)):
(:host):
(picture):
* css/html.css:
(@media (inverted-colors)):
(video):

LayoutTests:

* TestExpectations: Platform setting only available on Mac and iOS.
* accessibility/smart-invert-expected.txt: Added.
* accessibility/smart-invert-reference-expected.html: Added.
* accessibility/smart-invert-reference.html: Added. Ref to ensure invert and grayscale filters render as expected.
* accessibility/smart-invert.html: Added. Computed expectatons of filter property text values.
* platform/ios-wk2/TestExpectations: Runs on iOS WK2.
* platform/mac-wk2/TestExpectations: Runs on Mac WK2.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@226825 268f45cc-cd09-0410-ab3c-d52691b4dbfc