Bug 1632795 - Ensure invalidate_rendered_frame is not overridden by the frame being a no-op. r=gw
Bug 1632795 - Ensure invalidate_rendered_frame is not overridden by the frame being a no-op. r=gw

The RenderBackend can be sent an `invalidate_rendered_frame` flag to
indicate that the current rendered frame is invalid. This is useful
when the platform requires a render, eg when starting or resuming the
app on Android. Upon receiving this flag, the render backend will set
a variable `doc.rendered_frame_is_valid = false`. Later on it will
decide to skip rendering only if this variable is true, so by setting
the invalidate flag we should be able to ensure the next render will
occur.

However, the RenderBackend also tries to skip renders which it
determines are not required. Currently it does this by setting
`doc.rendered_frame_is_valid = true` if it decides the frame is a
no-op. This overwrites the previous value set by the
`invalidate_rendered_frame` flag, meaning webrender skips renderering
even though the platform has requested it.

This was resulting in the GVE app showing a black screen on startup,
and Fenix temporarily showing a black screen whilst opening a new tab,
because despite WebRenderBridgeParent requesting an invalidation
immediately on startup, webrender ignored that request until it
decided it actually had content to paint.

To fix this, the logic should be flipped. The value of
`doc.rendered_frame_is_valid` must be remembered across document
updates rather than defaulting to false. And instead of setting it
true if webrender thinks the frame is a no-op, we must set it false if
webrender thinks the frame is *not* a no-op.

Differential Revision: https://phabricator.services.mozilla.com/D72600

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/2760b8a14f17ac39b2b6cc1ac2d63f2f1d05bb77

servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender