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Adds a URL to the PaintPreviewProto. This is a simple addition that is
used to attribute the Paint Preview to originating from a URL. This only
applies to the root frame rather than any of its children.

Bug: 1030419
Change-Id: I59028fbebf3305cb52b04fca9c190712a2bb40f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1949280
Commit-Queue: Calder Kitagawa <ckitagawa@chromium.org>
Reviewed-by: Ian Vollick <vollick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#721546}