Changes output format to histogram for using add_histograms() on Chrome perf dashboard.

Change-Id: Idd4f601db27ee27306f3dd159fb5b4ef5a992c4b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1743062Reviewed-by: mark a. foltz <mfoltz@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Clifford Cheng <cliffordcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#686662}

testing/buildbot/gn_isolate_map.pyl

@@ -1572,7 +1572,7 @@
       "--also-run-disabled-tests",
       "-v",
       "--use-live-sites",
-      "--output-format=chartjson",
+      "--output-format=histograms",
       "--output-dir=${ISOLATED_OUTDIR}",
     ],
     "label": "//chrome/test/media_router:media_router_perf_tests",