Updating string for ExpectUniqueSample failure based on feedback

Some of the Edge engineers got confused by this string and thought their
histogram was missing from the xml file. I tested this string with them
and they understood the failure better.

Thought it might be useful for the entire project but feel free to
suggest other options or push back!

Change-Id: I6fa6aa98329e419ca6f3b687a42a683ba7bd6828
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2487691
Reviewed-by: Cliff Smolinsky <cliffsmo@microsoft.com>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Auto-Submit: Joshua Berenhaus <joshber@microsoft.com>
Commit-Queue: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819746}

base/test/metrics/histogram_tester.cc

@@ -41,7 +41,7 @@ void HistogramTester::ExpectUniqueSample(
   } else {
     // No histogram means there were zero samples.
     EXPECT_EQ(0, expected_count)
-        << "Histogram \"" << name << "\" does not exist.";
+        << "Zero samples found for Histogram \"" << name << "\".";
   }
 }