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Changes Url to URL and adds an -IsURLNewTabPage helper.

Change-Id: If80a6e41d9b2d74ed5449550b49de1cb670bed07
Reviewed-on: https://chromium-review.googlesource.com/c/1350191Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610937}