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Add tests to check output format of the |WriteAscii| and
|WriteHTMLGraph| methods for |SparseHistogram| class. Tests check that
all components of the graph graphical representation are on there
places: graph header, newline symbols, graph pillars with value
description.

Bug: 1006447
Test: browser_tests --gtest_filter="SparseHistogramTest"
Change-Id: I6ab33d7848a105c1c3353d1c3b5852fbb2456a71
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1980729Reviewed-by: Kush Sinha <sinhak@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Andrei Salavei <solovey@google.com>
Cr-Commit-Position: refs/heads/master@{#735378}