[root-layer-scrolls] Fix android fullscreen video

Android uses an "overlay" composited video layer when a video is
fullscreened. This layer is parented to the PaintLayerCompositor's
root_content_layer and the actual content (LayoutView) is detached.

With root-layer-scrolling enabled, PLC doesn't create a
root_content_layer and the top-most GraphicsLayer in PLC is that of the
LayoutView. This CL connects the video layer to the PLC's parent
instead. This is currently the outer viewport scroll layer. We also need
to make sure painting starts from the child of this layer, rather than
the PLC's root layer since the video layer is no longer connected to the
root. This doesn't matter for the video itself, however, the media
player controls need to be painted from Blink.

These changes exposed some brittleness in the compositor and the Android
fullscreen video layout tests were hitting the DCHECK in
ScrollbarLayerImplBase::CanScrollOrientation. This is because the
viewport scrollbars use the outer/layout viewport as their ElementId but
are owned by the inner/visual viewport. So when we disconnect the PLC's
content layers, we remove the ScrollNode in the compositor but the
visual viewport and its scrollbar layers live on. The rest of this patch
changes the visual viewport scrollbars to use the visual viewport's
element id.

TEST=virtual/android/fullscreen/video-scrolled-iframe.html,
     virtual/android/fullscreen/video-controls-timeline.html,
     virtual/android/fullscreen/full-screen-iframe-allowed-video.html

Bug: 711468,811024
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: Ic1c52e1853f14842ee3ae19119476b8945ecae17
Reviewed-on: https://chromium-review.googlesource.com/923397


Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#540226}