Make media controls keyboard accessible
As media controls are implemented as shadow dom, media element is a shadow host.
This patch makes media element keyboard focusable by removing HTMLMediaElement::hasCustomFocusLogic to make isNonKeyboardFocusableShadowHost and isKeyboardFocusableShadowHost return expected value.
This patch also makes media element in standalone media document keyboad focusable.
removed keyboard support in standalone media document to avoid drawing focus ring by mouse click. However foucus by mouse click is disabled by https://src.chromium.org/viewvc/blink?view=revision&revision=106876
So, we don't need to disable keyboard focus.
Contributed by firstname.lastname@example.org