Auto merge of #9969 - mbrubeck:quotes-036, r=pcwalton

Don't re-resolve already-resolved generated content

This fixes #7846, a failure in the "quotes-036.htm" test. Servo lays out this test correctly in its initial layout, but then messes it up in any relayout (whether it's an incremental or full layout).

The problem is that the ResolveGeneratedContent traversal is not safe to run more than once on the same flow. It mutates some GeneratedContent fragments into ScannedText fragments, but leaves others unmodified (in particular, those that generate empty content). The next time layout runs, these remaining GeneratedContent fragments are processed *again* but with an incorrect correct quote nesting level (because some of the surrounding GeneratedContent fragments are gone).

This patch ensures that each GeneratedContent fragment is resolved only once.

r? @pcwalton

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