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Summary:
This is a Hackthon week project and I reworked the PrewalkTree function to be more streamlined.

Please see the design doc at the following link for purpose of the project and other details.
https://docs.google.com/document/d/1D1MTNTaJnZjbmMZGMz0u150N3u5evEFSmJU-ou2vcFw/edit#heading=h.c0uts5ftkk58


In short, the original PrewalkTree() is as follows:
---------------------
// Returns the damage rect of the surface
gfx::Rect PrewalkTree(Surface* surface) {
	......

	// Gather all the embedded child surfaces into a flat_map
	FindChildSurfaces(&child_surfaces, ...)

	// Iterate through the child surfaces to accumulate damage
	gfx::Rect damage_rect;
	for (child_surface : child_surfaces) {
	    damage_rect += PrewalkTree(child_surface); // apply quad transform here
	}

	// ...do some other stuff

	return damage_rect;
}
-----------------

I modified it to:
-----------------
// Returns the damage of a surface
gfx::Rect PrewalkTree(Surface* surface) {
	......
	RenderPass* root_pass = surface->GetActiveFrame()->render_pass_list.back();
  	return PrewalkRenderPass(root_pass);
}


// Returns the accumulated damage of a render pass
gfx::Rect PrewalkRenderPass(RenderPass* render_pass) {
	gfx::Rect damage_rect;
	for (DrawQuad* quad : render_pass->quad_list.reverse())  { // in reverse order
	    if (quad is SurfaceDrawQuad)
		  damage_rect += PrewalkTree(quad_surface); // apply quad transform here
	    else if (quad is RenderPassDrawQuad)
		  damage_rect += PrewalkRenderPass(quad_render_pass); // apply quad transform here
   	}
   	return damage_rect;
}
----------------


Bug: N/A
Change-Id: I662e93b3f61cd7102c54ca4ba1c2183d6233ac0f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2175315
Commit-Queue: Jun Liu <yjliu@chromium.org>
Auto-Submit: Jun Liu <yjliu@chromium.org>
Reviewed-by: kylechar <kylechar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#770994}