The logic used by guest/host and sidebar frames to find each other in
`Discovery#startDiscovery` relied on traversing the frame tree starting
from `window.top` using `window.frames`. Since `window.frames` doesn't
include frames in shadow roots, this did not work if _both_ the guest and
sidebar are contained within a shadow root. The sidebar is always
contained in a shadow root created by the `<hypothesis-sidebar>`
element, so this simplifies to only working if the guest/host is not
contained in a shadow root.

For current use cases it is possible for guest frames to get a direct
reference to the sidebar frame they should be communicating with and for
the sidebar to get direct a reference to the host frame (its parent).
This avoids the need for `window.frames` traversal.

This commit leverages that by changing `startDiscovery` to take an
explicit array of frames to ping. Guest frames invoke `startDiscovery`
passing a reference to the sidebar frame and sidebar frames invoke
`startDiscovery` passing a reference to the parent (host) frame.

This fixes the following scenarios:

 - Hypothesis not completing loading if host frame is contained within a
   shadow root. eg. In the VitalSource book reader.

 - Same-origin guest frames loading Hypothesis after the sidebar has
   already loaded. See http://localhost:3000/document/parent-frame test
   case in client dev server.

 - Multiple sidebars attempting to connect to the same guest frame, if
   Hypothesis is loaded multiple times in different parts of the frame
   tree. See http://localhost:3000/document/multi-frames test case in
   client dev server.

This approach has some limitations:

 - Child guest frames which are not same-origin with the parent host frame
   are not supported.
 - Child guest frames which attempt to connect to the sidebar before it
   has loaded will fail to connect. This could be remedied by making the
   guest wait for confirmation of the sidebar having loaded before
   calling `connectToSidebar`. This doesn't affect the host frame since
   the sidebar pings that frame directly.

These limitations are acceptable in the short term but will be remedied
by a larger overhaul of inter-frame communication that is currently
being worked on.