We should suppress compositor updates while presenting on Android
since they aren't needed, and they do a fair amount of extra work.

Unfortunately hiding the view doesn't currently work, it
causes an assertion failure (!m_isHidden) in
DrawingBuffer::prepareTextureMailboxInternal() via FrameView.cpp:

        if (!RuntimeEnabledFeatures::slimmingPaintV2Enabled()) {
          if (view.compositor()->inCompositingMode())
            scrollingCoordinator()->updateAfterCompositingChangeIfNeeded();

Disabling compositing mode works but seems irreversible, the view
is frozen after exiting presentation.

Do this only when we're actually presenting (m_isPresenting
is true), see corresponding show() in forceExitPresent().
Otherwise the view may remain hidden for failing DON flow.

Here's my current attempt:

  // hide in beginPresent() if no external display:
  auto doc = m_navigatorVR->document();
  doc->view()->hide();
  doc->layoutView()->compositor()->setCompositingModeEnabled(false);

  // unhide in stopPresenting()
  auto doc = m_navigatorVR->document();
  doc->layoutView()->compositor()->setCompositingModeEnabled(true);
  doc->view()->show();

This isn't technically a blocker since the compositor bypass can
still work while the normal compositor is also running, it's
just inefficient.