Test sparse histogram graphical output methods

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}

base/metrics/sparse_histogram_unittest.cc

@@ -385,4 +385,38 @@ TEST_P(SparseHistogramTest, HistogramNameHash) {
   EXPECT_EQ(histogram->name_hash(), HashMetricName(kName));
 }
 
+TEST_P(SparseHistogramTest, WriteAscii) {
+  HistogramBase* histogram =
+      SparseHistogram::FactoryGet("AsciiOut", HistogramBase::kNoFlags);
+  histogram->AddCount(/*sample=*/4, /*value=*/5);
+  histogram->AddCount(/*sample=*/10, /*value=*/15);
+
+  std::string output;
+  histogram->WriteAscii(&output);
+
+  const char kOutputFormatRe[] =
+      R"(Histogram: AsciiOut recorded 20 samples.*\n)"
+      R"(4   -+O +\(5 = 25.0%\)\n)"
+      R"(10  -+O +\(15 = 75.0%\)\n)";
+
+  EXPECT_THAT(output, testing::MatchesRegex(kOutputFormatRe));
+}
+
+TEST_P(SparseHistogramTest, WriteHTMLGraph) {
+  HistogramBase* histogram =
+      SparseHistogram::FactoryGet("HTMLOut", HistogramBase::kNoFlags);
+  histogram->AddCount(/*sample=*/4, /*value=*/5);
+  histogram->AddCount(/*sample=*/10, /*value=*/15);
+
+  std::string output;
+  histogram->WriteHTMLGraph(&output);
+
+  const char kOutputFormatRe[] =
+      R"(<PRE>Histogram: HTMLOut recorded 20 samples.*<br>)"
+      R"(4   -+O +\(5 = 25.0%\)<br>)"
+      R"(10  -+O +\(15 = 75.0%\)<br><\/PRE>)";
+
+  EXPECT_THAT(output, testing::MatchesRegex(kOutputFormatRe));
+}
+
 }  // namespace base