Multiple iframes in same isolated process results in one of them not being drawn.

[mattwoodrow]

bb4da53

Multiple iframes in same isolated process results in one of them not being drawn.
https://bugs.webkit.org/show_bug.cgi?id=264356
<rdar://118034981>

Reviewed by Alex Christensen.

RemoteLayerBackingStoreCollection::willCommitLayerTree collects all the backing stores that were unreachable,
and includes that list in the transaction so that the UI process can ensure the buffer references are removed.

This changes moves that call until we've finished building all the transactions, so that were not considering
buffers only used in latter transactions to be unreachable for the first transaction.

It also adds a new 'willBuildTransaction' function. The set of backing stores that need display is a property
of the transaction (where we call prepareBuffersForDisplay, and then paint them), so we want to clear this
state per-transaction, not per rendering update.

* LayoutTests/http/tests/site-isolation/double-iframe-expected.html: Added.
* LayoutTests/http/tests/site-isolation/double-iframe.html: Added.
* Source/WebKit/Shared/RemoteLayerTree/RemoteLayerBackingStoreCollection.h:
* Source/WebKit/Shared/RemoteLayerTree/RemoteLayerBackingStoreCollection.mm:
(WebKit::RemoteLayerBackingStoreCollection::willFlushLayers):
(WebKit::RemoteLayerBackingStoreCollection::willBuildTransaction):
* Source/WebKit/WebProcess/WebPage/RemoteLayerTree/RemoteLayerTreeDrawingArea.mm:
(WebKit::RemoteLayerTreeDrawingArea::updateRendering):

Canonical link: https://commits.webkit.org/270399@main

127406d

Misc	iOS, tvOS & watchOS	macOS	Linux	Windows
✅ 🧪 style	✅ 🛠 ios	✅ 🛠 mac	✅ 🛠 wpe	✅ 🛠 wincairo
	✅ 🛠 ios-sim	✅ 🛠 mac-AS-debug	✅ 🧪 wpe-wk2
✅ 🧪 webkitperl	❌ 🧪 ios-wk2	✅ 🧪 api-mac	✅ 🛠 gtk
	❌ 🧪 ios-wk2-wpt	✅ 🧪 mac-wk1	✅ 🧪 gtk-wk2
	✅ 🧪 api-ios	✅ 🧪 mac-wk2	❌ 🧪 api-gtk
	✅ 🛠 tv	✅ 🧪 mac-AS-debug-wk2
	✅ 🛠 tv-sim	✅ 🧪 mac-wk2-stress
	✅ 🛠 watch		⏳ 🛠 jsc-mips
✅ 🛠 🧪 unsafe-merge	✅ 🛠 watch-sim		⏳ 🧪 jsc-mips-tests

[webkit-early-warning-system]

EWS run on current version of this PR (hash 127406d)

Misc	iOS, tvOS & watchOS	macOS	Linux	Windows
✅ 🧪 style	✅ 🛠 ios	✅ 🛠 mac	✅ 🛠 wpe	✅ 🛠 wincairo
	✅ 🛠 ios-sim	✅ 🛠 mac-AS-debug	✅ 🧪 wpe-wk2
✅ 🧪 webkitperl	❌ 🧪 ios-wk2	✅ 🧪 api-mac	✅ 🛠 gtk
	❌ 🧪 ios-wk2-wpt	✅ 🧪 mac-wk1	✅ 🧪 gtk-wk2
	✅ 🧪 api-ios	✅ 🧪 mac-wk2	❌ 🧪 api-gtk
	✅ 🛠 tv	✅ 🧪 mac-AS-debug-wk2
	✅ 🛠 tv-sim	✅ 🧪 mac-wk2-stress
	✅ 🛠 watch		⏳ 🛠 jsc-mips
✅ 🛠 🧪 unsafe-merge	✅ 🛠 watch-sim		⏳ 🧪 jsc-mips-tests

[achristensen07]

This is great, skipping test on iOS in #20176

[webkit-commit-queue]

Committed 270399@main (bb4da53): https://commits.webkit.org/270399@main

Reviewed commits have been landed. Closing PR #20120 and removing active labels.