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This patch refines selection-repaint-with-gaps.html
It tests selection painting and invalidation.
It used to output layout tree into DOM and confirm its text
dump.
However it also output pixel result so output png was
 strange: it contained painted layer tree text dump.
This patch refines the test to dump:
- *.txt for layer tree
- *.png for paint result only contains selected text.


Bug: 739062
Change-Id: I60e9b16f2061a77d50e5a65fd287981f63518dc8
Reviewed-on: https://chromium-review.googlesource.com/701938
Commit-Queue: Yoichi Osato <yoichio@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#506971}