[Telemetry] Add indentation to json_output & chart_json_output.

Rationale: ocassionally I find it useful to look at json_output to debug.
Indenting the file make it a lot more readable. The downside is this increases
the file size, but it shouldn't matter for small files like these.

An example Results.json with ~2000 values tested on local machine:
Without indent: 472K
With indentation: 692K

Review URL: https://codereview.chromium.org/940123003

Cr-Commit-Position: refs/heads/master@{#317175}

tools/telemetry/telemetry/results/chart_json_output_formatter.py

@@ -71,5 +71,5 @@ class ChartJsonOutputFormatter(output_formatter.OutputFormatter):
         self._benchmark_metadata,
         page_test_results.all_page_specific_values,
         page_test_results.all_summary_values),
-              self.output_stream)
+              self.output_stream, indent=2)
     self.output_stream.write('\n')

tools/telemetry/telemetry/results/json_output_formatter.py

@@ -53,5 +53,5 @@ class JsonOutputFormatter(output_formatter.OutputFormatter):
   def Format(self, page_test_results):
     json.dump(
         ResultsAsDict(page_test_results, self.benchmark_metadata),
-        self.output_stream)
+        self.output_stream, indent=2)
     self.output_stream.write('\n')