Fix scrollperf logging
https://bugs.webkit.org/show_bug.cgi?id=107589

Reviewed by Tim Horton.

The scrollperf logging had two issues:

1. It relied on a paint logging a "filled" event, but it's possible
for existing tiles to be moved into the viewport and filling it, so
we need to log from the scrolling thread both when we have unfilled pixels,
and when the last scroll revealed unfilled pixels.

2. On some pages, z-index:-1 elements behind the body cause the root
TileCache to have drawsContent set to false, so none of its tiles paint. In
that case, the check for a non-zero paintCount in TileCache::blankPixelCountForTiles()
was wrong; we don't think there's a way to have an unpainted tile in the tree.

Also fix the signature of blankPixelCountForTiles() to take references.

* page/scrolling/mac/ScrollingTreeScrollingNodeMac.h:
(ScrollingTreeScrollingNodeMac):
* page/scrolling/mac/ScrollingTreeScrollingNodeMac.mm:
(WebCore::ScrollingTreeScrollingNodeMac::ScrollingTreeScrollingNodeMac):
(WebCore::ScrollingTreeScrollingNodeMac::logExposedUnfilledArea):
* platform/graphics/ca/mac/TileCache.h:
* platform/graphics/ca/mac/TileCache.mm:
(WebCore::TileCache::blankPixelCountForTiles):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@140475 268f45cc-cd09-0410-ab3c-d52691b4dbfc