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When a RenderWidget is created for a RenderViewImpl, it may be
considered "swapped out". This means the main frame for this
RenderView's frame tree is not local, the RenderView exists to
hold some other local subframe, which will be composited and have
a RenderWidget. Thus the RenderWidget attached to the RenderViewImpl
is not actually used, there is no output from the remote main frame
in this tree. I like to refer to these swapped out RenderWidgets as
zombies.

During navigations, a RenderViewImpl's main frame may change to or
from being local, in which case the RenderWidget attached to it changes
to or from being a zombie.

When becoming a zombie, the WebFrameWidget attached to the
RenderViewImpl (wrapping the WebViewImpl) is destroyed, and this
marks the compositor as SetVisible(false) on the RenderWidget
(via the WebViewImpl).

When becoming alive again, a new WebFrameWidget is attached to the
RenderViewImpl, and the WebFrameWidget marks the compositor as
SetVisible(true) on the RenderWidget (via the WebViewImpl).

While a zombie, the WebViewImpl is also told not to allow any
visibility changes to be passed along to the RenderWidget. This is
because as the local subframe becomes visible with the Page, we'd
tell the WebView about this, to tell the Page. But since the
WebView now has a zombie RenderWidget, it shouldn't also tell that.

Note when the RenderViewImpl is created and initially swapped out
it creates a RenderWidget but nothing tells its compositor about the
zombie state of affairs, so it becomes visible, which makes it
acquire a Gpu channel and start its scheduler etc. Woops.

This CL rearranges things a bit.
- A new RenderWidget does not SetVisible(true) if swapped out.
- RenderViewImpl directly marks it SetVisible(false) when removing the
WebFrameWidget.
- RenderViewImpl *also* removes the LayerTreeFrameSink to drop the
Gpu channel since we don't need to be fast at becoming visible (the
RenderWidget is a zombie!)
- RenderViewImpl directly marks it as SetVisible(true) when getting
a new WebFrameWidget as the RenderWidget stops being a zombie.
- WebViewImpl stops calling SetVisible on the RenderWidget when its
main frame is not visible. In the future the RenderWidget should be
a member of the main LocalFrame, and thus won't even exist when the
main frame is not local.

R=dcheng@chromium.org, piman@chromium.org

Change-Id: I87a5d0dfeaaf08cb91c9348b26b4205db55b3a81
Bug: 894899, 419087
Reviewed-on: https://chromium-review.googlesource.com/c/1290140Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Commit-Queue: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602389}