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This patch introduces a performance test file "select-all-words.html" for
measuring painting selection.

See https://bit.ly/2SyNQS1 for profiling sample comparing legacy layout and
LayoutNG.

As of February 7, 2019, LayoutNG is two times slower than legacy layout.

Change-Id: I8c5fe8418715b7d0078f6f1186c4191a1fb7d850
Reviewed-on: https://chromium-review.googlesource.com/c/1457701
Auto-Submit: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629909}